Saving to the memory card
I had a similar problem on my XTI in conjunction with the battery grip. It only occurred when the batteries were low. I was not getting an accurate reading on the screen of the battery levels and it would freeze causing me to lose the shot and have to pull the battery compartment in and out. A lot of times it was just high battery draw (continuous shooting) towards the end of the battery cycle. If I pulled the battery tray in and out I could usually get another 30-50+ shots until they were completely drained and it would freeze up again. Your situation may be different though. I have not had the problem on my 60d yet however. Much more battery life on this sucker.