Undeleteable extra rests
Working on a piece, I had to do some complicated cut-and-pasting of many measures across several staves. Now almost every measure has the wrong value of rests in it -- some have two eighth rest, others have ten, etc. etc. and I can't seem to put them back in the measures where they go. My whole project is a mess. Is there a plugin or something that can fix this?
Screenshot: 
Comments
Select the measures and press Delete? Or enter new measures (Add / bars ...) and eliminate those corrupted. If there are problems attach your score here if you want.
Welcome.
I'm afraid your score has become corrupted.
It would be very helpful for you to attach a copy of the corrupted score for the development team to look at, and also give the exact sequence of actions you did just before it happened.
It is possible that Musescore has saved a backup copy of your score which you can access.
Instructions on how to do that are here.....
https://musescore.org/en/node/52116
Based on that image, it appears you're using an old version of MuseScore that unfortunately a) was known to cause corruptions like this and b) made it ridiculously hard to fix them. Updating to MuseScore 2 should make it fairly straightforward to correct those problems, and also prevent them from happening in the future.