For a brief, magical period of time, you could surf on birds in Counter-Strike: Global Offensive. As in, ascend into the skies on their tiny, feathered backs. Unfortunately, it broke the terrorist-fragging heck out of the game, so Valve had to take it out. But they added something fun to make up for it.
When players found they could stand on a flock of birds in Counter-Strike's remake of classic CS map Train, they inevitably started using it as a "boost," as seen in this video by Rasafel:
Given that CS:GO is a game of positional awareness and tense tactical maneuvers, Valve designs levels so that people can't see everything. For obvious reasons, bird surfing/boosting/whatever you want to call it broke that. Players could scan the level from their glorious winged perches, or even reach parts of the level they were never supposed to set foot on.
So now that's out, but as PC Gamer points out, that doesn't mean bird surfing is forever forgotten. Valve's immortalised the... unexpected strategy with this sign in the level:
It apparently reads "on bird not tread."
It's a fun consolation for one of the more amusing Counter-Strike glitches in recent memories. Still though, first sweeping changes to chickens and now, a year later, this? Counter-Strike's really gone to the bir — [sound of lighting striking]
— irds. Birds. Sorry, I stuttered. But did you hear that? I sure hope nobody got hurt.