Probably more efficient to summarize everybody's contribution than to respond piecemeal.

The key thing that I missed, and that Doug and DavidV pointed out, is that R8 is a solar filter. It doesn't pass much light, so any image of the ground taken through it is necessarily a very long duration exposure. The CCDs accumulated photons for a very long time, making them more vulnerable to getting zapped by high energy particles like cosmic rays. You wouldn't see the effect on the normal short-duration images.

This doesn't sound like an unusual phenomenon after all...just one that's infrequently produced by the cameras.

Hey Doug: You mentioned seeing this type of image much earlier in the missions. If you remember where, could you spare me searching through thousands of images to find them, and post a link to a sample or two?

Thanks for putting my mind at ease!