Fizz Ultimate on a Champion that's retreating doesn't cancel the retreat.
I'm not sure if it's intended or not, but there seems to be a bug with Fizz's ultimate, which if used on a Champion that's half-way retreated, it doesn't cancel the retreat and the ultimate activates in the enemy base then.
Champions: Fizz and any enemy Champion that's half-way retreated.
Location on the Map: Everywhere, you could say (Where it's usable)
Items and the Game Menu have no effect on this issue
It happened in my last recent game as Fizz, in which i ulted Fiora as she was retreating, and the ult didn't stop her retreat, which caused the ultimate to activate in her base