I'm attempting to replicate this bug, but so far my attempts have been unsuccessful. If any of you guys have the answers to any of these questions, I would appreciate it if you could respond.
On which game mode did this occur for you? (Ranked, Normal, Co-op vs. AI, Custom game? Summoner's Rift, Twisted Treeline, Proving Grounds?)
On which champions have you seen this occur? Who gained the kill from whom?
What items did the "killer" have? What items did the executed champion have?
Were you attacked by or near the support at any point beforehand? What abilities did they use on you? Was any form of cc applied by them?
Did the "killer" have any activated or persistent auras at the time?
Even though I'd have no idea what I'm looking at, it may be helpful if you uploaded your r3dlog from the games in which this occurs.
You can learn how to do that here.
Finally, if anyone is able to replicate the bug consistently, please make a detailed post here.