The dreaded Err99 and Canon digital camera’s… If you’re a Canon owner and have never encountered it, good for you. I first experienced mine in the end of June last year. After a long walk in damp woods, we tried to take pictures of our location. I got one test shot, crimped, and pressed the shutter. The shutter clicked, but no picture showed. Just a helpful screen telling me, that I got an error.
I tried everything that came to mind in the field (without a camera bag). Turn the camera off, take out the memory card, take off the lens, remove batteries, etc. And various permutations of the previous actions. None worked. Back at our friends place, I quickly googled the error and tried the options for debugging the problem that I found. None worked.
No LensRentals.com has published a good overview of possible reasons and their recommendations on what to do if the error occurs.
BTW, the last shot I got before the error occurred is in Life of Jalo. And despite my heavy use of third-party lenses, this time I was using our 70-200 f/4 L USM, so no third party issues in this case. The final result after the camera came back from service was a broken spring etc. in the shutter/mirror combination. Luckily, even though the camera was over a year old, I got it repaired under warranty as I hadn’t even taken 30 000 shots when the error occurred.