Here's what happens when you unknowingly move the white balance knob on your camera and take all your pictures for the day with a white balance setting of "when taking pictures with flash only." All my pictures had an unnatural greenish hue. Lesson for the day, look carefully at your viewfinder settings and, in particular, AWB is your friend.
Of course, when Photoshop is in your repertoire, you can take your raw file and fix the white balance pretty easily. I didn't have any pictures worth saving, but it's a good trick to know in case I did have a keeper.