I had a session this weekend and while I was shooting my Canon 40D kept freezing up and saying on the screen ERROR 99 or something to the affect of having to remove the battery and turning the camera on and off. So I did this but it would lock up 4 shots later. EXTREMELY frustrating. I can see why you need a second camera. Can anyone bring me insight on why my camera is doing this? Has this happened to you? What did you do to fix it? Thank you!!!