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

[spritey]

Dressing up as a lolly pop for Halloween... because Haunter will come and lick you.

[Nightmare Eater]

Join Team Rocket. They lose and can never do anything right. And that's only the games.

[OranBerrySandvich]

Stranding yourself in Cianwood without a pokemon able to learn fly.

[SirBlaziken]

Fighting Primal Dialga with a level 1 magikarp.
Challenging Machamp to a boxing match.
Taking a baby Kangaskhan from its mother.
Touching a Muk.
Trying to catch a pokemon with a Voltorb.
Damaging a Piplup, Prinplup, or Empoleon's pride.
Trying to open a Voltorb.

[spritey]

Eating a suspicious-looking ice cream cone with eyes and a mouth.

[Hahex and Oshawott]

trying to take over the world... that's just generally a bad mistake to make everywhere.

[Reshurott Inferno]

Stranding yourself in Cianwood without a pokemon able to learn fly.

Surf can be used as a 'rescue' option.

Mistaking stuff like Palpitoad and Marshtomp to be weak to Electric.

[Mondo Oowada]

Endangering an innocent 10-year old's life with a fire breathing lizard.

[ShiraBliss]

Catch a Vanillite.

[OranBerrySandvich]

Tell Minun and Plusle their routine was awful.
No person with a soul could withstand the amount of sad on the pokemon's faces without bursting into tears.
NOBODY.

[lets all go out for some frosty chocolate milkshakes]

choosing chimchar as a starter

[Aggron is awesome]

Hatching an egg without a bike.

Seriously, it's horrible. X_x

Not if you have a speed hack hahahahahahaha 

[Aggron is awesome]

Catch a Vanillite.

actually vanilite is really good when it evolves but it doesn'y have a very wide movepool 

[spritey]

Not listening to your parents.

[RioluRiku]

Making eye contact with people. they will yell at you and attack you with creatures.