Operations applied to wrong score?

I had a couple of files open and I noticed that some regular edit operations weren't working. For example, I'd select a note and try to delete it, but it stubbornly stayed in place. I thought maybe a restart would fix it, so I saved the file, but upon closing MuseScore, it prompted me to save changes in a different, seemingly unmodified file I had open. I switched to that file and noticed a few notes were indeed deleted, and the score became marked modified when I switched to it. I undid those changes and closed that file, but then when I tried operating on the original file, MuseScore crashed.