I've seen two.
One was a 17/3/15 Ezreal bottom who was out of control fed by a bad Akali. He had lifesteal + cleaver + trinity + frozen mallet, and killed four attackers bot with the help of a Leona. He then shot his ult top and blew away me (a not-so-sneaky) Lulu trying to cap top.
Also I saw a Karthus ult for a Penta on my own team, we got into a team-fight over top point which was uncapped originally. My team ended up capping it after the enemy had committed with Karthus on the point. We popped Garrison, and it ripped the enemy team up pretty bad, then Karthus blew up one of them trying to get away.
Karthus went into the top bushes and pressed R for a penta.
My best ever was a quadra, defending bot point with Ryze against many ill-fated turret divers. Unfortuantely, their fifth was killed in a jungle skirmish right before that, so I didn't have a shot then at a penta.