Worst mistake to ever make in the Pokémon world...

[Reshurott Inferno]

choosing Piplup as a starter

That is what you meant, right?

Angering a Samurott or a Reshiram. Samurott'll chop, slice and dice you, whilst Reshiram will burn you 'till nothing left of you exists.

[spritey]

Sticking your hand in water. Trust me, if a Shellder bites you, you won't evolve.

[MangaDragon]

I don't know if anyone brought these up...

Trying to kill Team Rocket(as you know, they should have died many, many, many times by now) or even worse that could put you in the hospital: Cutting (or in Seviper's case) biting off Jessie's hair. (That was a great episode).

[kr9q]

DON'T catch Zekroms Counter part. Zekrom is way better. Period.

[Nightmare Eater]

Listen to the above. Trust me, looks don't make up a strong pokemon.

[kr9q]

^ I made a mistake and fixed it.
Don't listen to him.

[Nightmare Eater]

So not catching Reshiram, the mascot of black, is the worst mistake to make?

[kr9q]

Yep. It is.
Mistake a Phonebook for a Pikachu with no tail, ears, feet, or arms.

[OranBerrySandvich]

Yep. It is.

Listening to this.
It's not that hard to catch it, and you could probably get something off the GTS if you don't like it.

[kr9q]

Joining every evil team. That will make them mad.

[Reshurott Inferno]

Catching Zekrom, except if you wanna complete your Pokedex, then trading it to other n00bs who think that Zekrom's better than Reshiram. Honestly, it's not. Reshiram is much more worth it.

[RioluRiku]

worst mistake. not catching one of the legendaries because someone told you it sucks. ~looks at above posters~

[Nightmare Eater]

Going insane after killing a legend because someone told you that it was the worst mistake ever.

[SirBlaziken]

Challenging the elite four with a team built with just level 1 magikarps.

[Jolteon825]

I would have to say making a Zekrom or Primeape angry.