This has actually happened before at official League of Legends tournaments (that is, a fairly serious bug in a serious match). I believe their policy in the case of a serious bug is to re-do the match, but I am not 100% sure.
It depends. If the bug was caught before game of record, you just restart the game. Afterwards, it must qualify as a game deciding bug. If so, the game gets scrapped and the match is redone.