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.
YMMV: Left 4 Dead 2
Abridged Arena Array: Dead Center is the most played campaign in co-op and VS and especially VS because the first level has ledges where a Charger can knock survivors off for instant kills. No Mercy, another commonly picked level in Left 4 Dead, also became another go to level when it was ported to Left 4 Dead 2.
For custom campaigns, most players stick to City 17, Suicide Blitz 2, and I Hate Mountains. More obscure or less popular maps tend to have less dedicated servers for them, if there's any at all.
Alternative Character Interpretation: There's more than just a few hints that Ellis's chipper personality and Cloudcuckoolander tendencies are signs that he's become completely unhinged by the zombie apocalypse, and is spending the game in what's essentially one long psychotic episode.
Base Breaker: The Cold Stream campaign divided up the fan base between people who find it refreshing and everyone else hating it for being crap compared to other custom maps and how their favorite campaigns should have been in the DLC instead.
The entire game itself is a base breaker. No one can seem to agree whether or not Left 4 Dead 2 is a better sequel or how the first game had a better atmosphere, amongst other things.
Breather Level: The elevator in Dead Center and the raft in Swamp Fever.
Broken Base: In spades! Bring up any comparison between Left 4 Dead and Left 4 Dead 2 and watch the fur fly as you see massive flame wars break out over which is the better game.
Left 4 Dead 2 also has a broken base in itself regarding certain content. The Cold Stream DLC has divided people into two camps; those who defend it for being something different and how Valve is supporting the mapping community by having a custom map as a part of DLC while people who hate the DLC blast it for Cold Stream being too boring or simply upset that their favorite custom campaign wasn't picked.
Breaking the base even further are rag dolls and legs. Left 4 Dead had survivors rag doll upon death and players could see legs for their character when looking down. Left 4 Dead 2 removed the survivor rag dolls so that the animations for the defibrillator would play correctly (survivors still rag doll if they lose their grip from hanging and fall to some place that you can't physically reach) and the legs were removed in order to ease up on the rendering costs and enable the game to have more zombies generated. For years, people demanded to have the rag dolls and legs back, but Valve isn't budging.
Left 4 Dead 2 in general is a base breaker that divided fans between those who like the sequel and those that hate it.
Cargo Ship: Just listen to Rochelle when she finds a Magnum.
Catharsis Factor: Using the Chainsaw Massacre mutation to tear apart a tank in moments or using the Gibfest on full automatic to mow down EVERYTIHNG, especially sitting at the end of The Parish is soooo relaxing.
Complacent Gaming Syndrome: Auto shotguns are still the most used weapon, despite the nerf it got in its ammo capacity, but AK-47s are another commonly used weapon due to its power. Its accuracy is mediocre but the gun becomes a powerhouse with a laser sight. Magnums and melee weapons becomes everyone's weapons of choice for realism mode since they're the only weapons that can still kill common infected in one hit and require no ammo.
Crazy Awesome: Keith, if Ellis's stories about himself and Keith are any indication.
Crowning Music of Awesome: In Left 4 Dead 2, RE: Your Brains can be played on the jukeboxes found in some of the campaigns. It signals a Horde attack right after the line "But here's an FYI, you're all gonna die screaming." The Horde's screams are right in time with the chorus that follows.
While defending from the stage of a rock concert, Left 4 Dead 2 sends you One Bad Tank with the tank theme turned up to METAL.
As well as Midnight Tank, which plays for the other Tank of the finale.
Demonic Spiders: The common infected. The mutation "Flu Season" turns the Boomer and Spitter into these, increasing the spawning cap for each to about four at a time and speeding up the Boomer from a clumbersome waddle to brisk walking pace.
The design on Nick's ring matches one of Francis' tattoos, hinting at either a shared history or a chance connection in a gang. Nick might have also been in prison ("I'm not legally allowed to own a gun."), which, while indicative of his past as a con-man, may support this theory.
Keith. Not helped by the fact that some of the graffiti in the first game is written by someone named Keith.
Fan Dumb: Hoo boy. While the fans were pretty much alright when Left 4 Dead first started, Left 4 Dead 2 opened a huge can of worms. Between people who think they know what it takes to improve either game or the series as a whole, flame wars over which L4D game is better, people demanding Valve to fix everything right this instant, people demanding Valve to add more content, demands for Valve to make their favorite mutation a permanent game mode, and people who tell everyone else to shut up since Valve has done no wrong, it's hard to find people who actually play the game and not complain about something in the Left 4 Dead series.
Despite people who swear Left 4 Dead 2 is the worst game Valve ever made and them complaining about anything in the game on the Steam forums time and time again, these type of people either just never leave or are seen still playing the game. Even if you call them out on it, they'll claim that they still like the game, despite all the complaining they do.
Not to mention, when Left 4 Dead 2 was announced, there was a "Boycott" of the game. Considering most people who were in the Boycott STEAM-group bought the game at launch...wasn't very successful, was it? Ironically, most of their fears were ultimately justified, making the end of the boycott more baffling. When the original game was given one final token DLC, it came alongside Left 4 Dead 2 getting the same campaign and a port of the first campaign of the original. Valve eventually worked to port the entirety of the original into the sequel, including the DLC and the survivors. This did not go without response from those who owned the original game.
There's also a loud opinion over Rochelle and how useless she is as a character. People either want her dead, claim she looks too ugly compared to Zoey, or should be replaced with Zoey (which they can with mods).
The fan dumb are also openly hostile towards newbie players, especially if there's a free weekend or a sale on the game that attracts new players. Why is there hostility towards newbies? Veteran players claim their games are "ruined" by new players who do not know how to properly play the game and said complainers would rather drive new people away instead of playing a private game where they don't have to deal with new players.
Freud Was Right: Coach on the Jockey: "It ain't right for a man to be ridden like that!"
Fridge Brilliance: So the theme of The Passing is that no one survives forever? Well, we have this uncommon infected called the Fallen Survivor we didn't use. Think it'll fit the theme?
Fridge Logic: The Midnight Rider's website mentions their latest album can't be found in stores because their record producer decided they weren't profitable enough, but they're clearly well known and have a massive stadium devoted to them in Dark Carnival.
Fridge Brilliance: The Peach Pit in Whispering Oaks is not a massive stadium, nor is it devoted to them. In addition, The Passing shows that they were set up to play a small club that could hold maybe 50 people. Playing the fairground and club circuits is usually reserved for little-known indie bands and for nostalgia acts like the Riders who can't fill arenas and amphitheaters. The Riders were popular in the 1980s, but have since fallen on harder times and now tour almost exclusively in the Southern United States. Coach suggests as much when he says he has all their albums, "even their new stuff that ain't no good".
With the revelation that being immune still means you spread the infection, it's possible that when you give Whitaker the cola, you're actually dooming him... not to mention, the virus is occasionally airborne, and Whitaker's hideout doesn't seem to be sealed. This assumes, of course, that Whitaker wasn't immune in the first place.
Right before he leaves you in the last level, Virgil mentions that he is heading back to pick up more survivors. At the end of the campaign the army destroys the only bridge leading out of the city as they continue blowing it to hell, meaning that, even if he and any other survivors make it back alive, all they will have to greet them is a ruined, bombed city with no escape from the remaining hordes of infected.
Game Breaker: The AK-47 can one-shot commons on normal difficulty, and unlike the SCAR and M16 it can be fired as a semi-automatic, one round at a time. In a game where ammo conservation is critical, that's a winning combination. Its main drawback is a lack of accuracy at long range... but add in Laser Sights, which basically gives you perfect accuracy, and you can demolish just about anything in short order. In fact, if you get your hands on explosive ammo, a full mag to the Witch will kill her without startling her.
Goddamned Bats: The Clown Uncommon-Common qualifies to a lesser extent. Though no harder to kill than any other normal Infected, if not dealt with, they can drag in a Boomer sized horde. Which in turn makes it more difficult to pick out the Clown who will keep attracting more zombies.
Let's not forget the Mudmen Uncommon-common, who can take more punishment than the average zombie, crawl rapidly on all fours (often resulting in them being confused for a jockey or hunter at a distance, if you even notice them) and blind the survivors with mud when they attack.
Hardhat infected in "Hard Rain" which take noticeably more damage than normal infected and are not drawn to pipe bombs, making trying to cover a limping escape with one of these far less effective as they swarm five at a time.
They're also only drawn to bile bombs that splash on other infected, due to the effects of a bomb hitting the ground using the same coding as the pipe bomb.
The zombies in the sequel also have this uncanny ability to rush at you and immediately run to your sides so you can't hit them, then you blindly turn to smash them while 6 more surround you. They also don't move out of your way when you try to shove them and run.
As of late, there seems to be a random bug in the sequel where after you start the finale, the rescue vehicles immediately arrives.
The "Headshot!" mutation makes zombies immune to all shots except ones to the head. However, they have an absurd tendency to get back up after their head has already been shot/bludgeoned/slashed off and keep attacking you.
Worker Infected don't follow pipe bombs OR bile bombs that don't hit any zombies, because they use the same coding.
There seems to be a bug with Francis using the M16 in L4D2 campaigns that feature the original Survivors: namely, when other players look at him, the magazine of his gun occasionally moves into the stock, almost always blocked from view by said stock and his arm.
Hilarious in Hindsight: Many people to this day hotly debate over the quality of Left 4 Dead 2 with many people stating Valve hired n00bs for testing the game's bugs and difficulty. The same people even went on to say that they could do a better job in finding all the bugs. Two years later, Valve releases the Cold Stream campaign and all the Left 4 Dead 1 campaigns in beta to the public so that players can test the maps and send feedback to Valve to help improve the maps.
Valve could be aware of it now. Check out some of Ellis and Nick's lines in The Passing.
Ellis:" Hey Nick, you and me outta get tattoos that says "Bros".
Or better yet:
Ellis: Nick, you have a turd on your shoulder. Nick: I hate you, Ellis. Ellis: I still like you, Nick.
Here's the paragraph on them from the Left 4 Dead Wikia:
Nick insults Ellis and makes fun of him on several occasions, but this seems to be more of a "joke" between the two of them. Ellis even seems to look up to Nick a bit, starting conversations with him such as having the same suit or sometimes being excited at Nick shooting the helicopter pilot. Nick, for his part, will compliment Ellis's shooting or ask him if the Midnight Riders are any good, which may be an attempt to bond with him. Though it may seem that Nick is less caring towards the younger man, he has a soft side too, like if both survivors (Coach and Rochelle) are dead, or if Ellis is dead he will sometimes apologize for his behavior. Ellis is always sad to see Nick die. However, Ellis is also angered by Nick when talking about horses, in the sense that Nick has a like of horse meat, and Ellis states that "horses are for riding" and not meant to be eaten. He also may sometimes call the Spitter "Nick's Mom!" Ellis does still see Nick as a very good friend, as in "The Passing", he may suggest that he and Nick get a tattoo with "Bros" on it, and will respond to Nick's exclamation of "I hate you Ellis," with "well I like you, Nick." Though Nick is rough around the edges, he seems to care for Ellis more then he comes across like (sometimes) if he heals Ellis he says things like "Don't tell the others this, but I'm doing this for you."
Not to mention the trauma Ellis gets when a survivor dies, especially when it's Nick.
Oh, and this lovely quote:
Nick: Ellis, did you get that girl's name? 'Cause I can tattoo it on your ass. She'll like that.
Plus, in the opening movie, they have their arms around each others' shoulders. To support one another, obviously, as they both seem to be limping, but still. Cue squee from the fangirls.
Let's not forget Ellis and Keith. It sounds like those two have a Brokeback Mountain past.
Let's also not forget Ellis's story about going with his friend, Dave, to a mall where men danced for money. Interestingly, the German version of the game replaces Dave with Keith. Germans love Keith/Ellis?
Though nothing compared to Ellis' reaction to Zoey, this bit from The Passing is certainly interesting:
Ells: Hey, Ro, now I don't wanna get your hopes up, but I think that awesome biker dude has a little crush on you. Rochelle: Well, I think someone has a crush on that awesome biker dude. Ellis: Well, I don't doubt it! That is a cool guy!
Hype Backlash: The Cold Stream DLC for PC players. Valve, towards the day of the release, announced some surprises they would reveal on release day. It turned out that the surprises was making Left 4 Dead 2 available to Linux users, having a Minecraft-themed community campaign be hosted on Valve's servers, and Valve allowing skins of the survivors to be used in the Xbox 360 version of Minecraft. Xbox 360 owners are pleased with all they will get, but PC players are left sour that the DLC contains no additional fixes for animations and vocals and there wasn't any new achievements made, despite the fact that Valve hasn't said anything about them.
Xbox 360 players also lashed against Microsoft and Valve after discovering the DLC for them would be delayed by a few days on top of the very long wait they had to endure since the beginning.
PC players also trashed Valve for picking Cold Stream instead of a different custom campaign due to Cold Stream being unbearably difficult at times, extremely linear, predictable, and the set pieces being very bland.
Ellis's friend Keith seems to have replaced "Chicago Ted" as a forum favorite. According to Ellis he has suffered from third degree burns over 95% of his body twice, broke both of his legs after driving his car off a cliff, been stabbed by a homeless man who he thought was a ghost, got tear gassed by the police, got attacked by an alligator, bombed by the military, drowned in the Tunnel of Love, got cut up by his own "bumper-car" lawn mower leaving him with wounds over 90% of his body, got cement paved over him in a sewer after falling down an open manhole, fell off a roller coaster onto the tracks with the ride still going...and lots of other things. He will likely be the last one left after the Zombie Apocalypse.
In the final campaign of Left 4 Dead 2, it is revealed that soldiers began shooting zombies and humans alike due to the possibility of immune humans being carriers of the Infection. The Survivors even found some dead soldiers who had engaged in a Body-Count Competition, with different markings for different targets.
Given the fact that the Zombie outbreak has pretty thoroughly wrecked at LEAST large portions of the continental US and utterly destroyed several major cities, it is fairly understandable why at some point in time the Joint Chiefs and the President dialed it in and decided to save the remaining half of the country by completely annihilating anything trying to get out of the destroyed half.
The original survivors probably cross this: they know that the army are killing carriers. They don't tell the L4D2 survivors this. In their defense, that wasn't revealed until the Sacrifice comic, which came out after The Passing. That might've not even been a story element when The Passing was made.
Narm: "OOHHHH NOOOOOO!" That's the sound of Bill being ridden by a Jockey.
Newbie Boom: Every single time the game goes on sale or had a free weekend, game servers would be flooded with new players, many which have zero experience or think their FPS skills from other games would carry them through Left 4 Dead 2 and greatly annoying veteran players in the process. This in turn causes veteran players to whine about newbies ruining the game.
Taken to another level around Christmas 2013 where Valve made the game free to download for a day. Yes, anyone who downloaded the game during that time got the game for free and would own it forever.
Rescued from the Scrappy Heap: Rochelle in "The Passing". While she had her fans, she had many haters before the DLC was released, due to her supposedly nondescript personality. But after it came out, most people did a complete 180 and warmed up to Rochelle thanks to her new lines in "The Passing".
Paranoia Fuel: The "Last Man Standing" Mutation. At least Virgil the talking boat will be there to help you soon.
For those playing the sequel, the entire Healing Gnome gametype. You have no permanent health, there are absolutely NO health items, and the gnome slowly recovers health...Of the survivor holding it. To make matters worse? Survivor bots will absolutely not pick up the gnome. Any survivor not controlled by a human player is no longer a survivor.
The melee fatigue system. Shove zombies too many times in a row and your character will tire out and have to wait before they can shove again. Introduced in the first game for VS mode in order to cut down on people who shoved zombies to death instead of shooting. It was then added as a standard feature in the sequel for all game modes, probably to encourage players to start using melee weapons, but it is still frustrating to be unable to shove due to fatigue and you lack a melee weapon to defend with.
Scrub: Left 4 Dead 2 has a ton of these when it comes to VS mode. Let's just say that if anyone tries to talk about the game's balance, you will mostly hear how the survivors are too powerful or how Congfogl is the more "proper" mode to play since they feel it's more balanced. You can also see scrubs come out of the woodwork in VS mode whenever a survivor kills them with the grenade launcher and then complain later on that the weapon should never spawn.
Self-Fanservice: Go look at how the Hunter, Witch, and Smoker look in the game. Pretty hideous looking zombies, right? Now go look how they are drawn in a lot of fanart. Notice any differences?
"The Passing" also gives us Francis and Rochelle, with Francis trying several times to hit on her and will even sob like a baby when she leaves. These conversations seem to have managed to have gotten Rochelle Rescued from the Scrappy Heap to some.
Before Left 4 Dead 2 came out people shipped Zoey and Louis. Whatever happened to that?
Before we even knew anything about the characters at all, people were shipping Nick and Ellis together, solely because they were both hot guys and on one poster it looked plausible that they were grabbing each other's asses. Zoey/Ellis is second only to Nick/Ellis in the fandom.
Ship Tease: The Passing had hints of Ellis/Zoey and Francis/Rochelle. However, it could be argued that anything between Ellis and Zoey was one-sided since Zoey never explicitly showed romantic interest in Ellis other than concern for his well-being.
One theory suggests that what the smoker shoots at you isn't his tongue - it's his intestines. The sequel slightly confirms this. Snapping a Smoker's tongue in Left 4 Dead 2 tends to spawn an extremely tangled up wad of 'tongue', and spawning in an improved Smoker in Garry's Mod allows you to see that it does, inside its mouth, have a normal-sized tongue.
Another theory suggests the same thing for the Boomer where the belly button would be. Again, generally confirmed in the second game, with the growths being a bit more... distinct.
Don't forget that theory concerning the Witch and her fetus claws!
Even if The Spitter doesn't get "spit" all over you, its looks alone might just make you puke. (Although she was originally supposed to be even more disgusting◊.)
Special Effects Failure: In "The Passing" campaign, Zoey had used Nick's skeleton model, stretching her arms to unusually bizarre proportions, which was fixed later on.
Another patch broke Zoey's model in the same level where instead of leaning on standing in place as she watches the other survivors escape, she stands in place and then suddenly slides across the balcony, resetting her pose into her default "standing with my gun equipped" pose. This has still yet to be fixed.
The L4D survivors also still lack their falling down animation when they are incapacitated, which means as soon as they're knocked down, they instantly collapse to the floor without any transition.
The L4D survivors have their melee weapons model become invisible (when looking at another player) when they are not in use and suddenly materialize when they are pulled out, making it look like the survivors have Hammerspace.
"Stop Having Fun" Guys: There's a good portion of the player base that looks down on anyone that doesn't play on Expert difficulty and even consider Expert too easy. The same players also tout that VS mode is the only good mode to play and how Campaign is boring while ignoring Survival and Scavenge mode.
More dedicated VS players will also play VS in a very specific way that can determine what special infected will spawn next and they will rarely stop to do a quick check in a side room for supplies when playing as the survivors. Naturally, these types of players frown upon anyone that dares to play VS differently.
The grenade launcher also invokes this trope at times, mainly from people who simply hate the weapon and think no one should use it just to avoid some friendly fire damage.
Stop Helping Me!: The survivor AI when it comes to healing. They will do their damn best to keep you going, but ignore anything else occurring at the moment. Even if that means stopping you where you're standing, potentially condemning you, them, or some other teammate to death. Obviously crosses over with Artificial Stupidity.
Not to mention how they persistently try to heal you the second your health drops below 40. They adjust their "judgement" a little depending on how much temporary life you have, but not very much. All those wasted med kits...
Tainted by the Preview: The game's reveal at E3 caused many fans to declare that the game was going to suck, based on the fact that the survivors didn't "look interesting" compared to the survivors in Left 4 Dead, the levels taking place in the daytime, how stupid the new special infected look, and the fact that the survivors can use melee weapons. The game still sold extremely well at launch and it surpassed the sales of the first game, though the Vocal Minority would tell you otherwise.
That One Level: The crescendo event of "The Barns" in the "Dark Carnival" campaign. Notable because there isn't a large wave of zombies like other crescendo event rather than an ENDLESS OCEAN of zombies that doesn't even let up until you get into the safe house. Also interesting to note that this is the chapter right before the concert finale and is notably harder if you aren't prepared.note Since the AI Director caps the number of zombies in play it is recommended to distract the zombies with a pipe bomb or Boomer bile, so stock up on those for this one.
A Chainsaw is also helpful in this part since you can just plough through the horde. Or any melee weapon, for that matter. Though even that can backfire; you could potentially be swarmed by so many zombies that you can't move at all.
Also the Sugar Mill in "Hard Rain" where up to twenty Witches can spawn. This is largely Schmuck Bait, as the witches are the "wandering" type and are harder to spook... But even with experienced players, a shot can go wild.
For many new players, the titular coaster in the Dark Carnival campaign is this. Especially if it's a group of four new and still incompetent players on a normal difficulty game. Rarely does one make it through if they're new to the game unless there are also experienced players in the team to guide them.
The finale of Swamp Fever has you fight two Tanks at once.
Swamp Fever is also disliked as a whole by a large group of people because there's a lot of water that makes you go a lot slower, the mud men cause your screen to get blocked and is less of a challenge and more of an irritant and generally doesn't have as good a level design as others.
The Sacrifice, in the final minute, has the Director summon FOUR Tanks at once. Though you don't have to kill them at all, you just have to hope they don't get between you and the generator. Unfortunately, they generally do.
Both Death Toll and Dead Air's fourth chapters in the sequel ports, with the addition of gauntlets—that is, infinite zombies that only stop if you reach the safe room. Or die.
Cold Stream's second level stages the gauntlet in a fast-moving current leading to an Inevitable Waterfall. Anyone incapacitated there must be left behind, because stopping to help them will get you yanked over the waterfall. And then its final level is a gauntlet the length of an average level, and the most geographically complex one in the game.
"Questionable Ethics." All of it. It's a custom campaign with holes in every ceiling so that infected can spawn above you and then drop down for an ambush. It's designed as a Nintendo HardPlatform Hell, with booby traps in random places and a 2nd-level finale that involves descending on a maze of pipes while cars fall from overhead. There is Suspicious Videogame Generosity right before the finale. ...And, in the room of the finale, three corpses: Zoey, Francis, and Louis.
To clarify on the traps and such, there are some paintings that colored yellow and knocking them over reveals either an item or a special infected in a wall. Purple paintings triggers a Tank and you have to hit them to proceed. Red paintings then start to show up, which alerts a horde should you hit it and later on, you not only encounter a room full of these triggers, but you also find another room that has alarmed urinals, toilets, tables, refrigerators and statues! Some parts of the floors in certain maps change into a glass square that shatters when you walk on it, causing you to hang from the ledge. You can simply approach this area slowly so the triggers happen early, but there's a second set of these traps later on that you are forced to rush through due to a gauntlet event. As for the pipe platforming mentioned above? The first pipe course can be taken slowly so you can watch where the cars fall (at least, until you wait too long and the pipes retract into the wall) but the pipe platforms appear in the rush to the ending in the finale and you have to work your way down while a horde and a Tank are hot on your ass. Bots are generally horrible for this campaign.
The sequel, Questionable Ethics: Alpha returns with its frustrations. The beginning of the 2nd level has you fall through a shaft that is littered with pipes and landing on them will kill you due to the massive fall damage. As you fall, you have to use the grenade launcher on the way down to shatter the glass floor and due to the wonky hit detection, your shots could wind up doing nothing and your body smacks into the glass, causing instant death. Naturally, the bots won't help since they don't use the grenade launcher. The finale itself takes all the gimmicks from the first Questionable Ethics and throws them at you in a finale where you have to keep running like you do in The Parish.
The "Detour Ahead" campaign is overall very balanced... except for the finale, which isn't a Difficulty Spike as it is a Difficulty Spire. Right after you get started on collecting fuel for a chopper to bail you out of the city, a plane crashes, and suddenly the entire city is after you: all of the time, you'll have a mob around the size of two Hordes hot on your trail, with more coming both from the front and the sides of the road. And they don't stop coming, even when the Tanks spawn. To boot, the finale is long, with the way to the fuel truck spanning about the length of the Bridge finale in "The Parish" – and you have to backtrack the entire way to the helo. Fun. You will be VERY lucky if you manage to escape with one bot... on Easy.
You can't help but feel somewhat bad for Ellis in the trailer when he's at the mercy of almost every special infected that appears. Or his absolute horror when he finds out his idol Jimmy Gibbs has been turned to a zombie.
Getting constantly picked on by Nick contributes to him being The Woobie.
This doesn't stop him from picking back.
Ellis: Nick, there's a turd on your shoulder. Nick: I hate you, Ellis.
They Changed It, Now It Sucks: The initial reaction of the community when Left 4 Dead 2 was announced. Complaints ranging from levels being set in the daytime, characters not being moody or grim like the survivors in the first game, new weapons making the survivors survive too easily in VS mode, and others.
And naturally, the same reactions again once Valve ported some of the Left 4 Dead 1 maps to Left 4 Dead 2 that included small changes for the new gameplay style.
Unwinnable by Insanity: In the scavenge finale of The Passing, the AI Director can spawn boredom hordes and specials if you take too long before talking to the first game's Survivors. If you do, activating the elevator will make the AI director think you have lowered the bridge, and he will begin throwing the infinite tanks and hordes at you... Even though you haven't lowered the bridge or even collected one gas can. And the best part? This doesn't reset if (when) you game-over. The only way to avoid it is to 1) quit the campaign and start from the beginning, or 2) not take too long before talking to the other Survivors.