My 18-month old Canon Rebel XTi (400D) started giving me the dreaded (Err99) error message about a week ago. I was taking some shots and suddenly the edges of the viewfinder screen went dark and the camera refused to shoot. The screen on the back said I had "Err99" and I should shut the camera down, remove the battery, put it back in and turn the camera back on. At first, I could get one or two shots before this happened, but finally, the camera refused to shoot at all. The shutter would go, but no image would be recorded. I was shooting with my EF-S 17-85MM f4-5.6 IS USM, but I put my EF 70-300mm f/4-5.6 IS USM on and the problem still happened. This tells me that the problem is in the camera body, not the lenses.
Searching the web, I found some suggestions on how to fix the underlying problem. I removed the battery and CF card, let the camera sit for 20 minutes and then put them back in and tried it. This didn't fix it. Then, I cleaned the lens contact points on both the camera body and the lenses with a white eraser. This seemed to fix the problem - the camera shot just fine. However, it now occurs to me that I tested it in a dimly lit room, rather than outdoors.
On Sunday, I was all lined up to get a nice shot of a DHC Beaver and it happened again - Err99. I tried the other lens and still it happened. I borrowed a pencil eraser and cleaned the contacts. Still I got Err99. I was on a ferry boat at the time and was so frustrated about missing the shot that I felt like throwing the damn camera overboard. I had trusted the camera and didn't even have a backup (my PowerShot S3 IS) with me.
Today, someone at work suggested resetting all of the camera settings back to default. This seemed to work and the camera shoots fine, both in dimly lit situations and outdoors in the sun. However, now I don't trust the beast. The question is: is it really fixed? Have other people out there had experience with Err99?