Editing produces an invalid bar
This is not the first time this has happened to me, and a couple of times I've had to rewrite the whole thing from scratch: I edit a note, and then suddenly I end up with a bar containing the wrong number of notes. In the attached file, you'll see I've added the 4/4 time sig on each line, hoping the program would right itself, but no. Bar 10 now has lost two notes and I can't reinsert them because there is only room for three in that bar.
I think this usually happens when I juggle (fight) with the size of rests, but I'm not sure.