If you are in NTSC or another interlaced format like PAL, you may find a damaged still with motion smearing. You can see these ahead of time by forcing your canvas to 100% with the tab on top and then pulling your canvas window larger and larger until there is gray around the edges. When you stop on a frame, that will show you both fields and warn you of danger.
There are ways of "fixing" a still with that kind of damage, but we'll take that after you succeed or not.
Koz