These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Terraria version 1.0.6 quickly became one of the most polarizing updates in it's history due to a few high caliber bugs that made it into the game.
One of the more annoying changes is the inability to place anything in lava anymore, meaning that making tunnels to Hell are much harder to create now unless you exploit dropping sand into the lava or are really patient about using water to make obsidian walls or digging a reservoir to drain the magma into. Since mining Hellstone creates lava now, this makes Hellstone difficult to mine without lava-immunity potions or digging a reservoir below to carefully drain the magma into.
1.2 however gave us boots that allowed the player to walk on water. If you somehow got your hands on Lava Charm and Obsidian, you could combine the the boots with Obsidian and then the resulting boots with the Lava Charm. That would give you not only the ability to walk on fire tiles and walk on lava surface, but also the ability to sink into the lava for 7 seconds. Equip a Lava Charm with these boots as well and you will be able to stay in the lava for fourteen seconds.
The release of 18.104.22.168 corrected the bugs and fixed the fireblossom issue, but it got broken again when 1.1 made it so not even the sand trick worked anymore.
The announcement marking the end of Terraria's development after one last bugfix has broken the base into those who think Terraria is ruined, those who just accept that all things come to an end ultimately, and those who believe that the real experience has just begun, and its time for Game Mods to shine.
Subverted; as of recently, Terraria 1.2 has been confirmed for July 2013... Until it missed such...
When the announcement was made that XBLA and PSN versions of the game were being developed and that they would feature new content, the majority of the old PC crowd were not pleased to say the least, especially because this news was hyped up the day before after months of silence.
Bone Serpents found in the Underworld. You are surrounded by lots of lava (which kills you very fast), and they have lots of health.
In the underground jungle, you have swarms of Hornets. They have 100 health and deal 40 damage per hit. 1.0.6 gave them a ranged attack. Hornets (and their big and little cousins) and Man Eaters found in the Underground Jungle. Hornets come in huge swarms, can shoot their stingers at you, and inflict poison. Man Eaters, meanwhile, are tougher versions of the Snatchers you find on the surface, and they're much more dangerous underground because you can't see them coming until they chomp you for 30+ damage.
The console version introduces Dragon Hornets, which are stronger than normal Hornets. Insultingly, they don't even have good drops, leaving only a pittance of copper coins.
Clowns only appear on hardmode Blood Moons, and very rarely, but they are this game's answer to Creepers, throwing bombs that can destroy terrain. Thankfully, 1.2 changed it so that their bombs no longer destroy blocks. The bombs still hurt like hell though.
Corruptors were this for those who like to make projects, as their spit used to corrupt terrain prior to the 1.2 patch. Corruptors are tougher versions of the Eater of Souls with a nasty ranged attack. Said ranged attack also corrupts terrain wherever it hits, helping the Corruption's already fast spread rate. They also have a small chance of inflicting the irritating Weakened debuff whenever they hit you.
Unicorns, which are faster, tougher and more powerful than most monsters.
Armored Skeletons in hardmode, which can inflict the Broken Armor debuff, which halves your armor rating for five minutes and turns even the weakest of Hardmode enemies into a serious threat. It's not that they're tough to beat. It's that they can inflict a five minute debuff that halves your armor rating.
Ichor Stickers, from the Hardmode underground crimson biome, for similar reasons. Their ichor debuff also reduces your defenses, in this case by 20 points. The good news is it only lasts 15 seconds, not 5 minutes. The bad news is that unlike Armored Skeletons, which have to physically touch you to apply their debuff, Ichor Stickers can apply theirs from extremely long range with their spit. Also its name doesn't really rhyme, no matter what the developers might think.
Clingers in the underground Hardmode Corruption. They shoot cursed flames, can move through walls, and tend to be near each other. Made even worse if a Corruption spot has spawned under your house. Clingers, tougher versions of the Man Eaters that spawn in the Underground Corruption. They can reach through walls, spawn in huge clusters, and spit cursed flames that don't go out when you jump in water.
Hornets and Man Eaters, found in the Underground Jungle. Hornets come in huge swarms, can shoot you from surprisingly long distances, and can poison you. Hornets are so dangerous, the game's wiki advises first-time players to wall themselves up whenever they see Hornets coming, and just kill them with the Vilethorn. Man Eaters, meanwhile, are basically a pair of mandibles on the end of a stick, and they can reach through walls to chomp you, leaving you helpless - and the Vilethorn does fuck all to hurt them.
Voodoo Demons, which are basically Demons with slightly boosted HP and the Guide Voodoo Doll as a drop. If you aren't careful when you kill them, they might drop the doll into lava and summon the Wall of Flesh. It's not that they're particularly dangerous. It's that, when they die, they drop the Guide Voodoo Doll directly underneath them - and the Wall of Flesh doesn't distinguish between intentional sacrifices and accidental sacrifices.
In Hardmode, a new type of demon called the Red Devil appears. They can fire 5-6 tridents that each deal massive damage note (120-150+ even with Hallowed Armor)! Thankfully, they're rare and their tridents are slower than Demon Scythes.
Giant Bats. They're everything bad about bats - fast, comes in huge swarms, hard to see, harder to hit - with insanely high attack power and a small chance of inflicting Confusion. Giant Bats are unusually dangerous for bat enemies. Despite the name, they're only slightly larger than the smaller varieties and just as fast, so hitting them with anything other than melee is difficult at best. They also have surprisingly high damage output unless you're wearing Adamantite or Hallowed armor, which you won't be for most of the time you're exploring the caverns. Even worse, they inflict Confusion, which reverses your controls and has led to many a death from accidentally walking into lava.
Literal spiders. One type occuring in the crimson which have high health and attack. Another variety spawns in caves among masses of respawning webs and tend to attack in large numbers. Both varities are very dangerous to early game players. In hardmode, the Wall Creepers get replaced by the even worse Black Recluses, which deal very huge damage AND inflict Venom, which drains your health much faster than Poison.
Necromancers in the Dungeon that appear during Hardmode. As soon as they take damage, they teleport and shoot highly damaging beams. The catch? The beams bounce off walls and are incredibly fast, making it incredibly hard to dodge and avoid being damaged. As if that's not enough, the beams have a chance of inflict a slowing debuff.
Their counterparts, Diabolists and Ragged Casters, aren't much easier to handle. Diabolists also teleport like crazy, but they shoot explosive magical projectiles with huge area of effect explosions. Ragged Casters fire rapid-fire magical projectiles that inflict the Blackout debuff, which is like Darkness but worse.
Skeleton Snipers can deal out tremendous damage from incredibly long range. Even worse, they tend to blend in with the mobs of other skeletons.
Spiked Slimes. Spiked Jungle Slimes have incredible accuracy, powerful attacks, and are harder to see than Hornets. Spiked Ice Slimes lack the range of their tropical cousins, but they have crazy high rates of fire and can inflict the Chilled and Frozen debuffs.
Pigrons turn invisible and fly through walls. While their damage is fairly negligible, they're nearly impossible to see until they hit you.
The console version introduced a lot of these, with many of them being reskins of previous enemies with boosted stats. Examples include Dragon Hornets, Arch Demons, Orcas, and Arch Wyverns.
Arapaimas, in the Hardmode Underground Jungle. No real tricks about them - they're just faster than you and really, really powerful. If you fall in the water and get swarmed, you're not getting out.
Moss Hornets are a pain to deal with in hardmode underground jungle. Their ranged attacks come out very fast and hit rather hard, particularly to a player who doesn't have upgraded hardmode armor yet, can travel through water, and can poison you. They generally have impeccable aim as well, so get used to jumping around a lot to throw them off. Melee oriented players without a decent ranged weapon will also have difficulty with them due to their tendency to fly away from you if you try to get close to them. They also like to attack you in groups and are extremely common.
Chaos Elementals are incredibly fast and can teleport at will, including directly on top of you.
Rune Wizards. They have some of the most powerful attacks in the game, and their projectiles can pass through solid blocks.
Sharks are fast, take off huge chunks of health with every bite, and can take some serious punishment. However, they're only found in the oceans, so you pretty much have to be trying to encounter them.
Goblin Sorcerers don't pose much threat to a prepared player, but they are excellent at killing your friendly NPCs by either teleporting into their rooms or hitting them with their projectiles. God help you if you don't have the Vilethorn.
The reason why one shouldn't spawn a surface Mushroom Biome before Hardmode (unless they're well equipped) is because the Mooks there have loads of health and deal very high damage. Special mention goes to the Mushi Ladybug, which hurts much more than the other foes there, moves very fast, has very high knockback resistance, and can fly for short distances.
The Giant Fungi Bulb is a constant threat in Mushroom Biomes. It behaves similarly to the Man-Eater, only it will also shoot spores that float erratically, deal a lot of damage, and can pass through solid objects. Because of these menaces, it's a good idea to bring something like the Death Sickle or Nettle Burst if you're wanting to explore Mushroom Biomes.
Epileptic Trees: A theory is floating around that the Angler is a ghost. Evidence supporting this is his claim of not drowning, but sleeping and his going poof when killed as opposed to everyone else's Ludicrous Gibs (for that matter he has a unique death message that says he leaves instead of being slain).
Fridge Logic: Bones can drop from Skeleton warriors in the dungeon, but not the random skeletons you find underground. Justified for game balance; bones themselves are a decent ranged weapon, and they can be made into Necro Armor, which is a ranged-based set of armor that increases damage and reduces ammo consumption.
Which may be explained by the skeleton's appearance. Its bones are green, and probably worn away from underground dampness or something.
Game Breaker: Originally, the Aqua Scepter sprayed a constant stream of water that did both continuous Knock Back and damage, with a very low Mana cost per second.
Before 1.2, the Hamdrax removed anything in one click. Justified in that it was literally the last possible tool to get in the game. The item lost its hammer function in 1.2, but it still remains a very effective tool for mining and chopping down trees.
Go to the underworld, create a line of something (like obsidian), put a lot of heart statues that're connected by wire to a one second timer that's switched on, you've got a setup that can allow you to solo all four hard mode bosses at once. Admittedly, you at least have to really work for this one, since heart statues can't be crafted, only found. And even with a 3rd party map viewer to guide your way, collecting enough of them for this to be viable takes a long time.
Another method of killing basically any boss that's a lot less resource- and time-consuming is to stick a single block of damaging ore in the boss arena of your choice, take off any items that protects you from being damaged by it, equip the accessory that increases the length of your Mercy Invincibility, summon the boss and grapple into the block of ore: since the ore does a lot less damage than any of the endgame bosses' attacks and damages you again as soon as your invincibility runs out, you're largely safe from most of their attacks and can just keep firing your Megashark at them at your leisure: furthermore, since you're stuck to the block of damaging ore thanks to your grappling hook of choice, none of the boss attacks that do manage to hit you can knock you away from it. However, this method doesn't seem to work too well on Skeletron Prime, mostly because his attacks have a tendency to linger on your character for a long period of time, greatly increasing the chance that you'll take full damage from them. He's also noticeably harder to hit than other bosses when you're confined to a single spot.
With the release of 1.2, the Spectre armor (which heals the user for a percentage of the magic damage they deal) becomes this when used with the Magic Cuffs (which restore Mana upon taking damage) and powerful Magic Weapons like the Shadowbeam Staff or the Magnet Sphere. The Hard Mode bosses can be taken down fairly easily when using this combo.
For melee characters, 1.2 adds the Vampire Knives, a melee-type throwing weapon that tosses a fan of lifestealing knives. You can score multiple hits and leech a lot of health back with each hit; couple that with the higher defenses of Melee armor builds and good accessories, and a melee knifer can solo Hard Mode bosses with some skill.
The Scourge of the Corruptor is a javelin that spawns two or three tiny Eaters when it hits a surface. The Eaters seek out and home in on enemies on screen. Combine that with the massive melee damage dealt by the javelins themselves and the incredibly fast rate of fire, and you've got a huge swarm of tiny seeking projectiles all over the screen.
The Flairon dropped by Duke Fishron can be seen as an upgraded Scourge of the Corruptor: creates high damage homing projectiles faster than the Scourge. However unlike the former it is unpractical to use in closed spaces: the Flairon requires space to create its projectiles, the bubbles doesn't last too long and they'll pop if any touches any blocks.
The Piranha Gun fires a mechanical piranha that latches onto enemies and deals constant damage as long as the player holds down the mouse button. Properly buffed, it trivializes bosses.
The Rod of Discord makes exploration trivial. Why dig when you can just teleport from cavern to cavern? Chug a Spelunker Potion and a Hunter Potion, and you'll be able to see everything around.
Nerfed in 22.214.171.124. It now requires ten seconds between teleport to avoid the Chaos State debuff. Teleporting while Chaos State is active causes massive, unblockable damage. It's also no longer possible to teleport into the Lihzahrd.
The Spectre Armor with the Lifesteal headpiece is normally not very powerful due to the 40% damage penalty, but if you stack damage buffs and get Mana Cuffs/Celestial Cuffs you can tank massive damage and heal at the same time without ever needing to pop a mana potion, especially if you have some powerful, constantly-hitting spells like the Magnet Sphere.
Goddamned Bats: As you get better gear, previously challenging monsters stop being so dangerous but due to the fact you get knocked back if you suffer so much as a single damage, they can easily become this. Even after you have a full set of molten armor you can still be killed by a lowly slime slapping you into a pit of lava. Unless you have a Cobalt/Obsidian Shield to prevent knockback.
Any enemy if you're trying to manage your items or craft something, especially worms. Even if they don't do much damage, they'll knock you around (and away from your crafting table) until you leave your inventory to kill them.
Want to get your hands on some Meteoric Iron? You'd better have the patience of a saint, since Meteor Heads will be constantly harassing you as you try to pick it up.
The game has literal bats, but most of these (save the ones in Hardmode that confuse you) are fairly tame. The actual Goddamned Bats are spellcasters — monsters that teleport at will to anywhere nearby, and can shoot their projectiles through the wall and love teleporting to places you can't reach without tons of digging. Thankfully, most projectiles can be swung at with a melee weapon or tool to get rid of before it hits you, but still. Wizard enemies, which teleport through walls and shoot projectiles at you (which also pass through walls). They're tough enough to take a few hits to kill, and they love to appear behind walls so you can't immediately kill them. Thankfully, their projectiles can be destroyed by melee attacks.
Fire Imps, the underworld variant, are even worse. The underworld is at least 75 percent lava at surface level, and all the good hellstone deposits require digging through the towers spaced between the lakes. This means the Imps often appear in out-of-reach places. They can and will show up when you're busy fighting Bone Serpents or mining. Ah, and unless you take preventative measures to have flooded the area with water, or obtained the Obsidian Skull, you'll also be busy avoiding lava puddles and molten hot ore. Have fun! Fortunately, the spawn rate of the underworld was toned down in a subsequent patch, although fire imps are still a pain if they start lobbing fireballs from an inaccessible location. Trying to mine Hellstone in The Underworld is especially annoying when Fire Imps keep pestering you every 5 seconds, especially when there's lava everywhere. Once you actually close the distance with one of these bastards, they die quickly and easily and tend to just stand there while you smack it to death. The only problem is that another one will spawn in just a few seconds....
There are actual bats in the game and they're just as annoying as you might expect. Their hit-box is a mere one block, making them ridiculously hard to hit with ranged combat, they do upwards of 20 damage per hit if you're not in high tier armor, which you likely will be when you first see them, and they like to swarm.
Harpies. Flying enemies that appear at high altitudes and use a high damage projectile feather attack that shoots out multiple projectiles.
A variety of bat in the game is the Hellbat being the worst of the bunch. While not particularly annoying by themselves, they can overtake beginner players quickly.
Then in 1.1 they added the Giant Bats, which are tougher and do more damage than the Hellbats. Oh, and they can also confuse the player, potentially making you walk yourself into lava or a crowd of Skeleton Archers. Or more Giant Bats.
Speaking of skeleton archers, they're annoying as heck too. The hit you from a long way away, do hefty damage, and can set you on fire.
A more potent example are the Lava Slimes in the Underworld. They jump far and fast and literally leap off of lava like a spring. They also used to turn into lava when killed.
After the newest patch introduced the Big Eaters and Little Eaters, it appears the number of Eater of Souls and its variants have skyrocketed. People have seen as many as 15 at once.
Gastropods shoot pink lasers from off screen with ridiculous speed and accuracy and deal massive damage for a common mob. Plus, they tend to gang up on you.
Chaos Elementals run like crazy, charging you and teleporting all over the place.
For a new player or someone setting out with a new character, Flying Fish are difficult to deal with due to their numbers.
Goddamned Boss: A number of bosses like to alternate between floating out of melee range and ramming the player to death, requiring the player to keep a projectile weapon and stay out of these bosses' range.
Fluid dynamics are completely broken, allowing you to create automated generators that create unlimited amounts of lava and water out of thin air, which in turn allows you to mine unlimited amounts of obsidian, which you can then turn into an unlimited number of obsidian skulls, which you can then sell for an unlimited amount of money. Basically, once you obtain a bucket of lava, your wealth is only limited by your patience.
Minimizing the program in a certain way causes it to run ridiculously fast in the background. This is handy if you want to park somewhere safe and wait for nightfall/daybreak, or if you've set up an automated monster-farming trap and want to wait for a bunch of Random Drops.
The differences between water and air pressure is ignored, so you can dive 300 meters underwater, dig a small tunnel, then dig up a bit into a roomy cavern, and the cavern will not get wet at all (to get the idea, imagine a J-shaped test tube that can fill all the way up on the right side without the left side overflowing).
During the Blood Moon, zombies can bash doors open, but only inward. Putting something that occupies the space but doesn't have collision (a cloth banner or another door, open in the opposite direction, for example) will make the door impossible to open inward, meaning that you can come and go freely but the zombies won't get in. It should be noted, however, that goblins don't have this problem as they bash doors completely off the hinges.
With autopause turned on, you can swing your sword and pause in the middle of the animation to click a potion up from your inventory, unpausing will consume it, even if you have the Potion Sickness debuff that would have prevented you from consuming it in the first place.
Light Discs, the Dao of Pow, and the Shadowbeam Staff, if used in just the right way, can actually glitch through one-block-thick walls.
After the 1.2 Big Update, it is possible for worldgen to create a Lizhard Temple in the Jungle with a messed-up layout that allows you entry into the Temple without having defeated Plantera and getting the Temple Key to unlock the door.
An interesting and much-exploited collision detection/clipping/zipping bug that was introduced by the sloped blocks in the 1.2 Big Update has been dubbed the "HOIK". Yrimir has demonstrated this bug by slaying the Dungeon Guardian with a HOIK setup that allowed him to attack it with impunity while zipping around it in a circle.
Examples which have been "fixed":
Prior to 1.1.1, trying to place a recorded music box on a table would duplicate the item by a massive amount, these boxes sell for two gold each.
Like all bosses the Eater of Worlds flees if he kills the player and there are no other players nearby to challenge him. Prior to 1.0.6 the Eater of Worlds was considered slain when he fled complete with full drops, allowing you to run back to the battlefield and collect its loot.
Early after the 1.2 Big Update, canny players were able to use Actuators to force Lizhard Bricks into the background, allowing access to the Lizhard Temple without beating Plantera and getting the Temple Key. This was soon fixed. Likewise, they used to be able to use a Teleporter to bypass the door. Both of these were 'fixed' by making it so that Teleporters and Actuators could not work in the Lizhard Temple biome until you defeat Plantera.
Version 1.2.4 added some amusingly overpowered acceleration mechanics with the Minecart's physics, allowing players to accelerate their carts to such an insane velocity that they can either get sent flying thousands of blocks away, accidentally get noclipped through terrain blocks out to the middle of nowhere, or sometimes even deal thousands of points of damage to any enemies or bosses their minecarts run through when blazing away at such ludricous speeds. This bug has been fixed as of 126.96.36.199, much to the disappointment of a number of players who were exploiting this trick.
Hell Is That Noise: The noise that the giant worms make when approaching you while you're alone mining can be particularly creepy, especially if you're using headphones.
And the mummies, which have a much deeper, more ominous groan than almost any enemy in the game, despite being very easy to kill. Even the light mummies with the ^^ expression on their face.
Junk Rare: Exaggerated with the Enchanted Sword, a rare sword with the ability to fire damaging energy beams with each swing. It is one of the most rare items in the entire game. The game has a small chance of spawning the Enchanted Sword somewhere underground when you first generate a world. There is no indication that the sword has spawned and since it's in a random spot underground you could potentially spend a thousand hours in a world with an Enchanted Sword and never know it's there. In the off chance that you do somehow manage to find an Enchanted Sword you realize that... it's terrible. It has only one point of damage more than the Light's Bane, a low tier weapon. The ability to fire energy beams isn't even all that special, even pre-Hardmode. The Ice Blade, another pre-Hardmode weapon, does the same thing, is almost as damaging, and is far easier to acquire.
Memetic Badass: Terraria player Yrimir has earned this reputation due to his exploits.
Moral Event Horizon: The Player Character. In order to progress to the endgame, you must summon a boss in the Underworld, and in order to do so, you must murder the Guide with the Guide Voodoo Doll. As of 1.1, it's clear that death is not cheap for the non player character allies, as when they die they are not resurrected, but rather replaced, and thus this amounts to murdering the Guide simply to progress in the game.
Painful Rhyme: The enemy Ichor Sticker was clearly given its name because the developers thought it rhymed. It doesn't. Ichor is pronounced with a long I sound followed by an "oo" sound ("eye-kor"), and sounds nothing like the word "sticker."
Praising Shows You Don't Watch: The game had a community like this pre-release, with many people praising the game well before it came out and claiming it to be nothing like Minecraft, despite only a limited number of them having played it at that point.
Rescued from the Scrappy Heap: Surprisingly enough, the Terra Blade. Even though it had extremely high power and Sword Beams, many people considered it a letdown for the effort needed to create it and consider it inferior to the Death Sickle, Paladin's Hammer, Horseman's Blade and Vampire Knives due to its lack of auto-swing. Cue Version 1.2.4, it got the auto-swing it deserved, and it's now back as a contender for best melee weapon in the game.
Since 1.0.5, he has become much more useful as he can show you everything you can craft with an item if you show it to him. Crafting is no longer a guessing game, and it certainly makes the world a whole lot kinder to new players who try to experience the game without immediately resorting to a wiki.
Lampshaded in the Steam card for him. "He thought it would be funny to open your door and let zombies in, you thought it would be fun to feed his soul to the lord of eternal nightmares."
As of 1.2.4, The Guide's Scrappy-ness is quickly being eclipsed by The Angler, who is the only child character in the game...which is a title he tarnishes by being an utterly infuriating Spoiled Brat. He's bossy and demanding, he treats your character like dirt, and is constantly pulling "pranks" on the other residents, such as putting undead fish into their furniture and piranha teeth into chairs. Even worse for those who want to give him the same treatment as The Guide, Infant Immortality is in full effect. If he is ever reduced to 0, he simply poofs away in a puff of smoke, with the chat log "<Name> Has left."
Scrappy Mechanic: Potion Sickness: whenever you use any item that heals your health, you need to wait a minute before you can use another. There's an accessory that can reduce it to 45 seconds.
Scrappy Weapon: Nobody uses short swords. Rather than swinging it overhead in an arc, your character thrusts it forward, making it almost useless against anything that doesn't walk right into it.
Squick: The Wall of Flesh, a gigantic... wall of flesh, with flunkies in the form of tiny mouths hanging off of veins that try to attack you. It can puke leeches to attack you with.
The Crimson is one big land of Squick. Blood drips from everything, even the trees. The enemies there have names like "Blood Crawler" (a giant tick/spider), and "Floaty Gross" (a ghost that constantly drips...some sort of unpleasant fluid), and you can farm things like Flesh Blocks (to build flesh furniture, of course), Vertebrae, and Meat Grinders.
The guide NPC who functions as a newbie hint guide. Unfortunately, he likes to path around the players and can enter their normally safe houses, bringing the monsters howling at the doors inside with him. NPCs running from monsters who have gotten into their homes can also run to your house and open the door that kept them out.
The baby slime summoned from the rare slime staff. It has Gameplay Ally Immortality and hits rather fast and can kill weaker enemies quickly. Unfortunately it can also negate your attacks due to the Mercy Invincibility the mobs have while being pummeled by the slime, thus your attacks don't always register. During hardmode, it quickly becomes obsolete due to its low attack power, and the higher defense hardmode monsters have. By that point you'll probably be better off killing things on your own rather than rely on the slime.
That One Achievement: The "Knight in Shining Armors" achievement on the console version. To get it, you have to obtain the ultra-rare Mining Armor, craft every helmet for the Harmode armors, get every other piece of armor, beat Ocram up to 30 times, then make 3 full sets of Hallowed Armor, plus 3 more of each Hardmode armor, and get a few hundred bars of Adamantite (the rarest ore in the game.)
That One Boss: Skeletron Prime if you're trying to beat him solo. He has four arms, each of which can do a fair amount of damage, and his head gains defense when it spins instead of losing it like the last version did. His high health and arms will soak up damage to near the end of the night, and if you don't beat him by then he becomes practically unbeatable and gets a One-Hit KO attack.
Wall of Flesh. If you don't prepare yourself in a ridiculous manner before the fight, you will have a LOT of trouble. Plus, the Hungry make ranged weapons a problem to use (unless you use Meteor Bullets or Unholy Arrows), and his incredible health pool and Beam Spam makes him a nightmare, possibly even worse than the above. NO strategy will assure success against that freaking wall. Which makes coming back with Hallowed Armor, the Megashark, Crystal Bullets, and Excalibur for round 2 (or whatever number fight it is) all the more satisfying.
Another reason why Wall Of Flesh is far more of a pain in the ass to fight than other bosses is the fact that he's by far the buggiest: he can spawn facing the wrong way which means that you either get grabbed by his tentacles right away and take a good chunk of damage without being able to prevent it, or if you try to maximize the area you fight him in by summoning him near the edge of the world, die instantly due to the Wall reaching the edge of the world right away. He can also spawn much further down than he's supposed to, making the world-spanning bridge you're basically required to build if you don't want to spend 90% of the fight swimming in lava or jumping over pits and structures useless. There's reason why the characters' page says there's no middle ground - you're either overprepared or dead.
How about The Twins? Sure, they don't seem so bad at first, until you get them down to half health. Spazmatism starts shooting a stream of cursed flames and trying to eat you, while Retinazer steps up the Eye Beams to Beam Spam levels.
The Pumpkin Moon. It starts out simple, with Scarecrows, Splinterlings, and Hellhounds that can be fairly easily handled with good crowd control weapons. Then you hit wave 4 and the Mourning Wood shows up. It's got 12,000 HP, it fires a huge spray of fire that sticks around on the ground for a few seconds and does huge damage, and it's surprisingly fast. It only gets worse from there.
The Frost Moon. It's The Pumpkin Moon Taken to Eleven The first few enemies like Zombie Elves and Gingerbread men, can be handled with ease with Pumpkin Moon loot, then you hit Wave 4 and the Everscreams come out with 13000 HP, being a harder version of the Mourning Wood. Later, you have to fight Santa-NK1 Which fire very powerful missiles and presents, that has 18000 HP. Even later, from waves 12 onwards you fight the Ice Queen with 34000 HP, which is pretty much Pumpking on crack, flying everywhere, firing ice shards. It gets hectic really fast, good luck getting past wave 12 on your own.
Duke Fishron introduced in 1.2.4 version. He's extremely fast, powerful and has 50,000 HP to boot, fighting him in a small-medium sized arena is downright suicidal as he will create huge tornadoes that not shoots high-speed, high damaging sharks in a very fast rate, but will also limit the player's movements. Think it can't get any worse? It Turns Red when low on HP becoming even faster and more erratic.
The Destroyer has become notorious as the most difficult of the three mech bosses, ever since it got buffed. It was briefly nerfed in 1.2 reducing its defence, damage, and health, but then 1.2.3 came along and reversed the nerf big time, raising its stats even higher than they were before. Additionally, there's less healing for you during the fight (since probes now only drop hearts 33% of the time instead of 100%) and it's completely immune to all debuffs!
That One Level: The Underground Jungle. It's a humongous cavern that stretches from below the Jungle to the Underworld. What's so hard about it? It's a nightmare to navigate, it's swarming with Hornets and Man Eaters (horrendous Demonic Spiders), and it's got several rare and rather useful items hidden in it. (Enable hardmode in 1.2 and its difficulty is boosted to Bonus Level Of Hell.)
The post-Plantera Dungeon, by and far. Originally it's dangerous but survivable and by the time you reach hardmode and have high end equipment it's basically a Breather Level. Then you defeat Plantera and the crap hits the fan. Suddenly the Dungeon is filled with extremely powerful skeletons that can soak damage like nothing, Necromancers that can bounce nigh-instantaneous shadow bolts around corners to hit you and teleport away whenever they take damage, Diabolists that shoot exploding, un-blockable fireballs through walls at you and also teleport on damage, Paladins that have more health than some bosses and rapidly throw hammers at your face for massive damage, Skeleton Snipers that can two-shot you from across a room. Basically every enemy in the post-Plantera Dungeon is either a Demonic Spider or a Goddamned Bat designed to distract you while the stronger enemies kill you. Even after you kill them you're still not safe because there's a chance every enemy you kill might spawn a Dungeon Spirit which charges you at high speed, straight through walls! The good news is that if you can survive this onslaught you can get some of the most powerful equipment in the game here.