In Windows 95, the most heavily-tested computer game was DOOM. Not because the testers spent a lot of time developing test plans and test harnesses and automated run-throughs. Nope, it was because it was by far the most popular game the Windows 95 team members played to unwind.
It was a huge breakthrough when DOOM finally ran inside a MS-DOS box and didn't require booting into MS-DOS mode any more. Now you could fire up DOOM without having to exit all your other programs first.
I've learned that in Windows Vista, the most heavily tested game was World of Warcraft. Most members of the DirectX application compatibility team are WoW players, in addition to a large number in the Windows division overall.
So if you have a wildly popular computer game for the PC, you can be pretty sure that the Windows team will be all over it. "For quality control purposes, I assure you."
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