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Error 99 is a generic message used to indicate a variety of problems, but mostly difficulty in communicating with lenses. Try taking the lens off and if necessary gently cleaning the contacts with some IPA and you may also need to remove the battery for a few minutes to reset. I've only seen that error once on my 20D and that was immediately after a lens change. Reseating the lens, battery out, wait and back in corrected the problem in my case.
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