This happened only recently. My sidebar.php file isn't long but it's surely longer than the editing window in admin. I was updating a portion of it, clicked "save", got "file updated successfully". Viewed site again the bottom part of the sidebar has gone. I repeated that and found that the whole length of sidebar wasn't delivered to the editing window of my browser hence this problem. Therefore if I have edited it and clicked saved only the truncated version is saved.
This may be a browser issue, has anyone experienced this before. Am using FF 1.5.04
I hope there is a plugin to backup theme files like database backup plugin, does anyone know?