One possible workaround is to do a Save As to a different name and then a second Save As back to the original name. Doesn't always work, but quite helpful when it does. If you are saving to a network location, you may be able to just Save As locally and then Save As to the network.
I got this message today on an assembly. I tried Jerry's tricks with no luck. I ended up fixing the issue by inserting the assembly into another, dissolving the subassembly and saving as a new file. I subsequently deleted the old file and renamed this new one to the old name.