Wards. We all buy them, or at least we should. We place them at the keyspots of a map to gain vision and that enables possible ganks and buff steals. Yet wards have another use (apart from eating your gold)! They have the ability to interrupt projectiles!
How does it work? It's pretty simple:
You are playing and suddenly a Rammus charges at you from the bushes with his Powerball. You won't survive if he Taunts you and you won't be able to get away if he bumps into you. So what do you do? You place a Ward in front of Rammus and he bumps into the Ward while you can happily flee.
As demonstrated in the video below:
Just go to the min 9 and see how its works ;p