The calendar I made is a workout schedule with links to exercise videos on YouTube. For the past year, I've been copying previously used links (which I have written out in a specific way) to future dates when I plan to do them again. But just recently it has started inserting the video itself into the cell, which messes it up like I said. It would be a lot more work to do everything manually instead of just copying things like I was able to do before.
(Like I said, using ctrl+z does fix it, but it's annoying that it happens in the first place, and I would hate to find one day that undo doesn't work for some reason, or I mess up somehow and have to manually try to put the calendar back the way it was, which is difficult because there's no ruler to let me see how big each cell is.)
This seems to be a recently added "feature," but I think it would be better to be able to go to insert > video if you want to embed a video instead of having it do it automatically when I copy a text link that leads to a video.