When I open my sidebar to edit it, a previously working site broke. I went back in to look at the sidebar.php code and the last 15-20% of the code was missing. I ended up restoring from the original sidebar.php from the theme and restored the modifications I had made.
Then I noticed something troubling. Sometimes, when I go into Presentation >> Theme Editor and select sidebar, I need to scroll down to check to see if the ENTIRE code has appeared before making any modifications. Sometimes I get partial listings. If I don't check to ensure that the entire code is there first, and make any changes and save, I'll end up with broken code.
If broken code appears now, I'll re-click the Sidebar link until I get a complete code listing, then edit what I need and save and all is fine.
I'm using FireFox 188.8.131.52.