Valve opened up the source code for Team Fortress 2, and the community responded with a slew of bug fixes and pull requests.

Team Fortress 2 updated with a whole bunch of fixes thanks to the community having the source code
Back in February, Valve released the source code for Team Fortress 2 (TF2) into the Source SDK, which allows modders to see how it all works and get their standalone mods onto Steam. But also, it allows them to fix up issues directly in Team Fortress 2.