I have a few although 1 was actually a bug, others were exploits.
I was playing support & my adc was literally right on top of me as we both moved into an unwarded bush where a support Fiddle was hiding. He used Dark Wind & insta-killed us both even though we were at full health. I'm thinking because we were so close together, it bounced an infinite # of times in a split second killing us.
I also remember people configuring their game config files to allow them to have Viktor's hextech item as a recommended item (which allowed them to buy it). It allowed them to cast Viktor's stun anytime they used W even though they weren't playing Viktor.
During that same day, I saw a Singed using an exploit to no clip through walls. It was actually hilarious watching people trying to get away from him.