My video playback application started crashing randomly on me a couple of days ago. I had initially chalked it up to a dodgy video files, since I'm rather used to having video decoders crash on marginal data. However, after it kept happening on some known-good files, I attached a debugger and broke into the crashed application.
And then promptly swore when I saw d3d9d on the top of the call stack.
See, I occasionally do a bit of 3D graphics, and when doing so, I set Direct3D to run with the debug runtime and the "break on D3D error" setting enabled. This is great when doing D3D development, but not so good when running a video player that uses 3D to display video and occasionally makes some minor transgressions. It hasn't occurred to me that the setting I was tweaking was also screwing over the video player parked in the corner of the screen.
Of course, this isn't the first time I've done this -- Titan Quest went from painfully slow to only moderately slow once I'd realized my mistake. I also had something similar happen when I had forgotten that I'd installed the DirectShow debug runtime and couldn't figure out where the timestamp display in the middle of the video player was coming from.