Leeroy Jenkins - WOW

I don't play WOW whatsoever, but some of my friends do. Anyways, my older sister had this on one of her tabs and I decided to watch it. I didn't really know what was happening and I didn't really see what was so funny about it until later on in the video, but this video cracked me up like hell. I mean I wouldn't stop laughing and I still don't understand why that guy did what he did. xD