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.
Demonic Spiders: In Angry Birds Epic, it's the Witch Doctor in Great Cliffs - 1. He's ridiculously hard to take down since he has a large pool of health, and his attack is enough to kill a single bird, therefore healing him a lot.
Also from Angry Birds Epic, the Shogun enemies. They deal a good amount of damage, have a high amount of health, and have an ability that allows all enemies a 50-50 chance of dealing a critical strike, meaning double or even triple damage. In addition, they have the passive ability that blocks low-level damage. This is made worse by the fact that they tend to appear alongside Ninja enemies...who have the ability to dodge high-level damage.
Ear Worm: We sincerely apologize if you have the game's theme music stuck in your head after reading this article.
Not just the theme music, but even the intro (that plays at the start of each episode) tends to get stuck in some peoples' heads too.
Fridge Horror: When you think about it, those birds are actually committing suicide… at least in-game.
The fact that the pigs intend on eating the eggs that they steal, even if the birds keep beating them to death.
Fridge Logic: The pigs stole your eggs. So you use white birds to drop more eggs on them?!
Maybe they’re unfertilised ones, or just not actual eggs?
Another one: You have to launch the birds at the structure to destroy it and get the eggs. The birds implode into nothingness (die?) upon doing so. So who's going to take care of the eggs once all the birds killed(?) themselves doing it?
Game Breaker: Terrance the Big Brother Bird is a pretty powerful bird that can break through wood, glass, and stone pretty easily, but thick structures stop him in his tracks. Combine him with the Super Seeds/Power Potions introduced in Angry Birds Friends and he's pretty much an unstoppable behemoth that can break through ANYTHING with ease, almost making it like a miniature version of the Mighty Eagle. The only real downside is that the Super Seeds are limited, meaning if you want more, you either have to get one randomly each day or buy some.
When it comes to hunting for hidden objects that unlock special levels, the purchasable Tusken Raider in Star Wars II works wonders. Simply launch him anywhere, touch the hidden item on the level, and it's yours in an instant.
The green bird, to the point where its awkwardness is lampshaded in a cutscene from Seasons.
Purchasing "The Mighty Eagle" and attempting to get "Total Desctruction" on every level of Seasons becomes problematic when you reach the Chinese New Year levels and the Eagle is replaced with a Dragon, which has a completely different pattern of movement.
The marmosets' ability to regain their balance in Angry Birds Rio, sometimes even after you've blown them sky high.
Serial Numbers Filed Off: The developers reportedly ganked the physics engine used in Castle Clout and Crush the Castle.
Sidetracked by the Golden Egg it's easy to get so caught up trying to get the bonus level by hitting the Golden Egg (or any other hidden item) that the actual level has to wait.
Stop Helping Me!: If you choose to restart a level that you plan on completing normally without the use of power ups or any "Mighty" characters, the notifications suggesting the use of a power-up (or in the case of Star Wars II, other characters) to pass the level can become this. Especially if you're working for the perfect cascade to happen in a level.
That One Level: Just how many times have you wanted to curse the level designers while attempting to get three stars on any, just ANY level that goes into this category? The number of such levels is pretty huge, in fact, and it's a lot easier to lose count than rather list them all.
Particular example. Level 3-5 stands out as being one of the most Googled levels, and that should really say something. Heck, a lot of World 3 counts.
In terms of collecting stars, almost all of the levels in "Mine and Dine" (when in some levels, a score of 100,000 still gives you 1-2 stars) and some levels, such as 2-12 and 3-1, are incredibly frustrating.
There's a particularly ludicrous "Mine and Dine" screen where the only way to get a three-star score is to launch a bird backwards off the screen and activate its special action blind.
Golden egg 5... You can go for hours ending up with one pig left every time.
They Changed It, Now It Sucks: A very minor example, pertaining to the win screen. In the 1.0 version, the area completion tune played after winning each level. Later updates would change this to a generic win screen similar to Rio, with no music, appearing after completing one level. The area completion tune plays when you beat an episode though.