I took some pictures with external flash yesterday, and found that the camera was set to custom white balance. I thought that it might be because of the flash sending white balance info, but a quick check revealed that that was not the case.
Aha! I bet Canon did it when they cleaned my camera at the balloon fiesta! So I reviewed the images and it just happened at random in the middle of shooting, after the cleaning.
I have no idea how it happened.