I have been into this trouble a few days now. While remote shooting in the studio through my laptop and the usb connection of the camera, I get this message on the LCd of the camera. Then I have to remove the battery to make it work again. This happens only when I'm connected and with a frequency of every other shot. Any ideas why is this happening?
You might also have a bad USB cable... It happened to me in un-tethered mode (not connected), so your issue might be different. Does the 99 error only happen with a specific lens?
err99 is a very vague error message.... it could be anything from lens contacts, low power, mirror box malfunctions..... and the folks at canon can't tell you what the problem is unless they have it in their hands. Fun huh?