I was working on an assembly file.  I've been saving every step of the way.  At the last bit when I was trying to close the file and save this error pops up:

How am I supposed to save my file when I can't even close it?

There was an option to Save all or don't save at the end...save all still gives the above error.

The only option left is to do don't save which I had to do to close the file.

What can I do the next time this happens?