Free Voidstaff.
This bug, has existed for as long as the undo button itself has. What irritates me most about this bug, is how long it has existed. I've reported it several times so hopefully I can get some awareness this time around. I've personally used this bug before in real games, I discovered it by accident, and I'm sure it has been used against me in real games too. It has literally existed for years, and it needs to go.
Description: When you "Undo" the Voidstaff, for approximately 8 seconds, you still have 40% Magic Penetration. This effect is reset with any single attack, however, the first attack within this time frame, will act as if it still has 40% Magic Penetration from the Voidstaff, despite the fact you used Undo, and were issued a full refund of the item. It is 100% reproducible, current to the patch we are in, and can be used on any Champion.
Video: http://plays.tv/video/5a812e925b3f180791/free-voidstaff