I've tested this on my 6D and my friends' 60D.
Use case: [Only occurs in Video mode] 1) Activate the IS function during Video mode. Does not have to be recording (but is reproducible in the same manner if recording). 2) Stop triggering the IS by letting go of the shutter button OR just start recording the video. 3) IS auto-shutdown after a few seconds. 4) Mirror snaps down; Camera displays "Ensure a lens is attached" error message immediately after IS stops (you can hear the IS stopping).
Very irritating since I now can't use my 70-200 to record any video unless I turn IS off.
Does not happen when I have the LCD on in shooting mode or any other situation. To have my lens serviced at a Canon repair depot would cost me $280 (default baseline price), which is a bit of a bummer since I don't even know what's wrong.
I bought this lens used from a previous CPS member in San Francisco when I was intern'ing there and the lens truly looks like it's in great shape. It was last serviced in Nov. 2011 and had a back seal replaced. Has anyone attempted a fix or teardown on this lens?