So basically, the title. It didn't seem to need any special circumstances or anything
Fiora R'd me as Teemo three times in a game and it would hit and damage twice but then put Fiora behind me while still untargetable for a short moment.
Tried it again with Fiora vs. Rammus. Same results. Ultimate would begin, 2 hits, then the animation would cease, she was behind me and unable to be targetted.
These were noticed on Howling Abyss.
My friend posted a video of it for the sake of the PBE, also, he's salty because he loves Fiora. lol