I have a bone to pick with Wurmple. Not with the Pokémon itself (I think it's kind of cute), but rather, with the way it is presented in Pokémon Ruby and Sapphire.
In Pokémon Ruby and Sapphire, Wurmple is one of the first Pokémon that you can find in the wild. You'll see it in the very first route, and so you're very likely to catch one at the beginning of the game. Later, your Wurmple will evolve into either Silcoon or Cascoon. You will probably assume that there is a logic behind which one it evolves into. If you played the game when it came out in 2003, you were essentially on your own figuring it out: all the nice online databases of Pokémon information were in their infancy, and - at least in Italy where I lived at the time - it's not like Internet access was that widespread anyway. So, if you were a scientific-minded asshole like me, you'd start catching all sorts and manners of these bugs and experiment to figure out what is that makes them evolve into the one or the other cocoon.
Almost immediately, I ruled out gender. I started thinking it was about day and night - after all, the final stage of one branch is a butterfly, and the other is a moth! Except that Ruby and Sapphire don't have a day and night cycle. The game does have an internal clock, though, so maybe you'd start thinking that there must be some day and night mechanic, except not as visual as it was in Gold and Silver...but that is also debunked. In my last desperate attempts, I tried to force Wurmple to evolve at an odd or an even level (who knows what I was thinking by then).
But none of these methods seem to lead to a definitive answer.
So, as for the actual answer. As I learned much later, Wurmple's evolution depends on its personality value, which is a number between 0 and 4,294,967,295 that is assigned to each individual Pokémon. Behind the scenes, this number controls things like the Pokémon's gender, its nature, its ability, whether it's a shiny or not, and a few other things here and there. Now, you can't see this number anywhere in the game, so, for all intents and purposes:
Wurmple's evolution is random.
There wouldn't necessarily be something wrong with that, but my problem here is: Wurmple is one of the first wild Pokémon you find in the game. You see it evolves into two different things. If the previous Pokémon games have taught you anything, you're going to assume that there is a rationale to how it evolves. There's always been one, after all. Some are harder to guess than others, but when you get it, they make sense. Eevee will become Espeon by reaching maximum happiness and leveling up in the day, and Umbreon by leveling up in the night. Okay. Tyrogue's evolution depends on whether its Attack or Defense stat is higher. Okay. Tricky, but okay. You can figure it out by playing, if you try hard enough.
And this is my point: for the first time, you're given a Pokémon where you simply cannot figure out what it is that governs its evolution. You cannot, ever. The number is invisible, no characters in the game give you the slightest hint about it, and, technically, the players should not even be aware of the existence of this number in the game's coding at all. The only reason we know about it is because of people hacking apart the code of the game (probably to figure Wurmple out).
But the main theme of the Pokémon games is to learn more about all the Pokémon in the world! You assume that Wurmple's evolution is one of the great mysteries of the Pokémon world, and you're given this puzzle at the very beginning of the game, so you set out on a quest to figure out it works.
Except there is no rationale. It's invisible and random. It's a beginner's trap. It's a trick question. So, if you fell for it, you only wasted a lot of time...and got pissed at this game.