My project had mixed sources, so one of them ended up with interpolated frames. (24 vs 30fps). In the video preview, the draft just shows the source without the processing, so I was startled when I took a snapshot and it was blurry from mixed frames. Eventually the reason became obvious and so I changed the project to match that clip and then took my snapshot (reverting my project settings later).

But... isn't there an easier way to get a frame out of a source? I looked in the trimmer but didn't see snapshot-y things there.