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

[the bread dragon]

never let thundurus-therian use nasty plot
you are just asking for it

[DisturbedHaxorus]

How about putting on a Stantler costume during hunting season?

[SirBlaziken]

Letting Garchomp set up in your face with SD and substitute.

[Pam-the-Lamb]

 Taking your bro Electrode onto an aeroplane.

It's too big for one seat....

[Captain Jigglypuff]

Trying to re-enact the Free Willy jumping scene with a Wailord that misses the ocean....

[SirBlaziken]

I'd say using an Aegislash as a melee weapon is a terrible idea.

[Delicious_Scout]

I would rather say that being used as a melee weapon by Aegislash is a worse idea.

[SirBlaziken]

^Could be.

I'd say trying to use a fire stone, water stone and thunder stone on eevee at the same time would be terrible.

[Captain Jigglypuff]

Pulling on random people's faces to see if they were really Looker in disguise

[Petzbreeder]

What about pulling on Cilan's face to see if he is looker?

Both have the same voice actor in the english dub.

Ironically enough, in the Capacia island UFO episode, Ash pulls on Cilan's face!

[Captain Jigglypuff]

Commanding Wailord to Body Slam a thief.

[Bulbakip]

All those are made according to Pokémon entries so if you dont know them none of those will have any sense- if you want to check them I'm writing in breckets from which game:

1. Making a Slugma stay on one place(Gold, HearthGold, Black, WHite, Black2, White2, Y)
2. Sleep near by a Drowzee (Silver)
3. Saying "your face is ugly" to a Yamsk (Black)
4. Hold a Driffloon as a balloon (HearthGold) (Black2)
5. Pass a Cacturne in the desert (Sapphire)
6. Grab Honedge's hilt (Y)
7. Kill an Yetal (Y)
8. Stop spoink from bouncing (Ruby, FireRed, LeafGreen, Diamond, Pearl, Platinum, Black2, White2, X, Y)

And a lot more I guess...

[Neotenens]

Ask a Duskull to babysit
Try to correct an Alakazam in math class.
Throw away a stuffed animal in front of a Banette.
Let a Garbador anywhere near your house.
Bring a Snorlax grocery shopping.
Refuse to make a kissy face for Giratina.
And finally, pet a Slugma.

[SirBlaziken]

Trying to beat Sawk in a karate match
Trying to beat Throh in a judo match

[Mockery for One]

I'd say mistaking a Shaymin for a bouquet.