https://pokepast.es/7c0d220ec973395eI went against a guy with a Ditto, had a brain fart and used Clangorous Soul anyway, and he sent Ditto in to copy my omniboost.
I thought I was fucked, but went for a Clanging Scales (since I knew Aura Sphere wouldn't work) and my opponent went for an Aura Sphere. I swept the rest of the team.
At the time, I thought I stumbled upon a Ditto killer, since I'd be forcing the player to guess what Kommo-O's ability was and I'd be able to kill it for sure if my opponent guessed wrong.
But it turns out that when Ditto transforms, it shows you which ability you got, and my opponent was just retarded.
What are some other sets that you thought would be good, but then disappointed you when you realized the mechanic didn't work the way you thought?