In a bit more detail - I've been tracking down a graphics driver bug the last few days. And this is like the easy case - it's on Windows XP and it's happening on my dev machine. I remember the old days of hell at Eclipse/Wild Tangent where we'd get a report that something in the graphics screws up, but only on Windows version XX and only with graphics card YY and only with driver version ZZ and only when you run Media Player at the same time, and we can't repro it any other way.
At WT we had a huge room full of like 50 computers with all kinds of different hardware and OS setups. Any time we did a release it had to be tested on all those boxes. (this was back in the day when you had 3d-only cards like the Voodoo and NVidia had just come out with the Riva 128, and you had to support all kinds of weird ass cards, in comparison the differences between even the most different of cards these days are very minor). I'm actually amazed how many PC game development studios don't seem to have this kind of test room.
We also had 2 full-time employees basically running testing all the time. I'm amazed at how many game studios have zero internal testing.