messed up screen sharing

I am screen sharing from an iMac 10.6.8 looking at a MacBook Pro 10.8. Quite often, but not always, the shared screen looks like this on my iMac.

(This image is anonymized.) So there is the real shared screen down below, and an inverted piece of it at top. Neither piece is active, in that I can't do anything on them remotely. What's going on? Is a display setting messed up on one machine? Help! This is really annoying. I can easily do screen sharing to other machines.