I've been watching all the add cover art edits for quite a while now, keeping an eye out for things like MBS-4636. One of the problems/patterns I've noticed in the last couple of weeks (at least 10 times so far) is that a user will accidentally submit two edits for a single image - the first one works fine, the second one is broken.

I've tried to reproduce this and I've asked users what they think they might have done that caused it, but so far I've not had any luck.
I've been trying in several browsers too, in case.

I tried pressing back and submitting the form again. That gave me an ISE about duplicate keys.
I tried refreshing the page after submitting the edit. Nothing unexpected happened.
I tried clicking on the submit button multiple times. That still only submitted the image once.
Someone suggested they might have interrupted the upload and submitted the form again, but I don't think that's it. The edit isn't submitted until after the upload is done, the broken image always comes after the working image and I can't actually seem to submit the same form multiple times.