Ok, dunno if it's a glitch, but:

Twist Mountain, inside caves, my Swoobat against wild Gurdurrs.

Certain Gurdurrs' attacks are super-effective while they shouldn't be and vice versa. Example:

Wake-Up Slap is super effective (Fighting on Psychic/Flying)
Rock Throw is normal effective (Rock on Psychic/Flying)

But maybe it's related to abilities in some way?