In a match i recently played an enemy Cassiopeia was able to exploit a bug where her W was completely invisible to everyone. The person entered the match with the intent to exploit the bug so it is possible to replicate it. I was unable to get any pictures since it was invisible.
I wouldn't exactly call that "game breaking". Sounds like more of a nuisance than anything.
A very low damage, AOE, slow that is invisible could be rather irritating to play against but I would hardly call it "game breaking".
I wouldn't exactly call this a game breaking bug, but this is probably something you could take a video of for proof. I'm pretty sure Riot's aware of some of the bugs where champion abilities turn invisible from time to time, with I think invisible Nidalee spears being the biggest offender because of their massive damage. They haven't really officially announced this anywhere but I read in an interview with one of the Rioters that they have been working on changing a lot of the game's back-end coding lately, so new bugs like that can pop up from time to time. I can't remember who it was an interview with, I just remember them going on a really long analogy about starting a business selling pies and expanding it into an pie empire. It was basically saying how things get harder to keep track of as the game grows
