Hitbox glitch had been in the game since launch
Have you played Team Fortress 2 at any point since it launched? Then you've probably been affected by this bug, a minor hitbox issue that somehow snuck under the radar for an entire decade before someone noticed it and Valve issued a fix.
The game shipped with this bug back in 2007 before it was finally fixed in a game update yesterday. The bug was simple, if you started a game as a Scout, Heavy or Sniper and then switched to one of the other classes, there would be some hitbox jankiness resolving in your getting killed unfairly or dodging hits. Similarly, start as one of the other six and then switch to Scout, Heavy or Sniper and you'll experience a similar effect.
The bug was reported to Valve by Reddit user SigSegv_ after he saw the bug in a video by the developer of the TF2 mod TF2 Classic, Nicknine, uncovered evidence of the bug in a video a few weeks back. SigSegv_ then made his own video to try and learn more about the bug, and it seems the exposure to the bug led to Valve fixing the issue, something that had probably gone unnoticed since the launch.
Sigsegv_ has claimed that fixing the code is as simple as changing one line of code, but finding which piece of code and where to find that was the real challenge. We've pinging Valve for a comment, and will update this story if we get one, but otherwise, developers should take heart in the fact that if Valve can't find all of their bugs, you've got an excuse the next time one of yours slips into a commit.