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

[1794mew]

forgetting whos in which egg =)

Adding to this: Breeding a whole mess of eggs, having them all spread out in your boxes then decide "I know I'll have an egg box!", move them all to the egg box, then realise you had bred the pokemon you were about to, then spend the next few hours trying to find which egg it was.

[Luc]

Forgetting to breed one of my best pokemon partners in pokemon white.

[spritey]

Trusting a Mawile.

[CheeseROFL]

Putting a Seviper and a Zangoose in the same room? ._.

[spritey]

Dressing up like a Durant and going to Victory Road where there are Heatmor.

[EagleEye101 : Impactz]

Forgetting that Ghost pokemon take no Damage from Hyper Beam...

Really this stuffed up my chances of defeating the elite four..

It make me sad :c

[ShiraBliss]

Mistaking a male Gardevoir for a female and try to make it breed with your Gallade.

[spritey]

Putting Grimer inside someone's soup.

[RioluRiku]

go into a pokemon village....
yes they do exist. if you go there they......will kill you

[spritey]

Swallowing a Poison Barb.

[Reshurott Inferno]

Nicknaming a Growlithe/Arcanine "RK9" in Gen ll.

[Happy Mask Man]

totally guilty.

Try to kill a shedninja with a tackle attack

A much worse idea would be to peer into the hole in Shedinja's back. I know a guy who did that once. Now he's a vegetable.
Throwing a doll away would also seem to be a bad idea.

[spritey]

Going near Darkrai and falling asleep...

[mr.taco]

1. Leaving Whitneys gym with out talking to her after you beat her
2. Looking all over sinnoh for the key that opens the house in canalave city

[CrazyBarks]

Nicknaming a Growlithe/Arcanine "RK9" in Gen ll.

Blamechuggaaconroy4that!