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

[JES]

Or anything that is a bug type, imagine Hypno shrieking at a Wurmple.

Oh, how I wish this forum had a "like" button like on Smogon!

Or Hypno fainting when one of my Scizors sneaks up on Hypno, and asking him "what're doin'?" before Scizor even has a chance to do anything!

[Kitt]

Giving PokeDoll to a Banette. WARNING: It will hate you forever if it loses its Pokedoll. The doll might end up attacking you as a Shuppet.

[SirBlaziken]

I'd say using Celebi to travel back in time, change one little thing and travel back is a bad idea, mainly because Celebi would cease to exist and you'd be stuck.