Video game bugs are like a foul odour. The cause might be some tiny thing, but if you can't isolate it, it will stink up the whole dang place. Such was apparently the case with a Team Fortress 2 bug that took Valve ten years to finally eliminate.
Here's how the bug worked: if you joined a server as one of five classes — pyro, demo, engineer, medic, or spy — then switching to scout, heavy, or sniper would mess up your animations and hitbox. So basically, if somebody took a shot at you, there's a chance it'd miss even though it sure as hell looked like it hit. In a game where The True Measure Of A Man comes from one's ability to make bullets go into other players' faces, it was infuriating. This issue had apparently been around since TF2 came out in 2007.
Here's a video of the bug in action, courtesy of sigsegv:
Yeah, it's pretty bad! It's amazing that this wasn't taken care of sooner.
A recent patch, however, finally took care of it. In Valve's patch notes, they wrote, "Fixed an animation bug that would cause the client and server hitboxes to become out of sync." TF2 player sigsegv, who reported the bug after it was recently discovered by TF2 Classic developer Nicknine, called attention to the change, prompting many people who weren't even aware of the issue, to go, "Ohhhhhh, that's why I missed all those times."
What did Valve have to change, though? Where was the domino that caused the whole game to fall over? Apparently, it involved the way characters' pose parameters were listed in the game's code. Pyro, demo, engineer, medic, and spy had theirs ordered one way, and scout, heavy, and sniper had theirs ordered slightly differently.
So there you go. Now if you miss in TF2, you'll have only yourself (and maybe also server lag) to blame.