If Emacs warns you about something, please stop what you're doing and understand what it's trying to say.
"File on disk has changed, do you want to save?"
This while remote editing the file. Blithely continuing, I hit C-x C-s and managed to corrupt the file.
In the split instant of saving the file and going "hmmm...who could be editing the file?"
ack! my colleague who was fixing a small typo .
Desperately pounding C-g, C-g, C-g.
No dice. I had a truncated file!
Good thing I had taken a backup the day before. A few calls and sheepish explanations later, sanity was restored.
If Emacs ever tells you something, listen.....carefully.