I recently purchased the 40D kit with the 28-135IS lens. The lens works great on my 40D but today I tried it on my 10D and I immediately got an Error99 message. I've tried other lenses on the 10D and they work fine (primes & zooms). Subsequent tries of the 28-135IS now don't produce the Error99 message but the aperture setting reads "00" and can not be changed in any shooting mode with this lens attached.
Is this a known problem with this lens/camera combo. Is it a camera or lens issue?
I used the 28-135 IS with my 10D for 4 years, never got a single error. Like others have said, probably a contact problem--maybe a little wear and/or finger grease on the mount contracts.