First time I've seen this error before... we were having network issues so I suspect it was related to that.
I'm kind of thinking that when they wrote this error message (and obviously the Revit programmers have put an awful lot of thought into this message) that it was a spelling error and should really say "Wait"... or "Please wait while I try and figure out what to do next"..!
Can you please add a little more detail...??!
Kind of reminds me of the joke where the radio operator of a sinking vessel put out the mayday call stating that they are sinking... and received a reply asking what they were thinking about?
But once again I digress.....
I did read one blog after doing a search for this error message and found this...
" I am still trying to figure this one out, are the programmers hacking my webcam and calling me fat? Or is my building going to somehow throw off the earths gravitational spin if the model is actually built?" http://bdmackeyconsulting.com/revit-weight-warning/
That made me laugh!!... but then again this project is a high-rise.. just in case I'll ask structural what this building weighs....
Filing this error message under "Ridiculous".
Update: Ok, so we got this message again and I couldn't remember how we got around it last time... this time we couldn't even close the file...
So we blitzed (deleted) the back up files, saved and this time the sync worked!