Heya guys, quick PSA.
Teemo and Karthus have historically been disabled in the One For All game mode (their core fantasies create a degenerate play pattern for the opposing team, that goes above and beyond what the other champions can accomplish), but this morning they were accidentally still enabled for a few hours. We've since disabled them in the One For All queue.
You can still use Teemo & Karthus in custom games if you want to though, on the map of your choice! :D