Why isn't my lens working with the Canon Rebel XT?

Question from Ben Trumbo: It seems like the Err 99 message is fairly common with Canon's Rebel line of dSLRs. Furthermore, it's a sort of catch-all error for many different problems including lens errors, battery problems, and memory card problems. However, the vast majority of those reporting the problem seem to think it has to do with their lenses. There are some very extensive discussions of the problem here and here. The latter link suggests a fix (cleaning the lens mount contact points), but as you'll see from the comments it doesn't work for all users.

Answer from Ben Keough: The problem here is that if it's a lens problem, Canon's customer service people will probably tell you to contact Quantaray and ask them if their lens is compatible with the 350D or not--chances are Quantaray hasn't tested it or will refuse to give you a clear answer one way or another. You can try to send the 350D back to Canon for a repair, but even if they accept it as a defect and repair it, you might have the problem again. That seems to be the experience that a lot of users have had, anyway.

I'd attempt the quick fix suggested above and see if it works. If not, contact Canon and see what they have to say about it. Beyond that, your guess is as good as mine. Good luck!