I definitely HAVE seen this very nasty problem. There is a 2 GB limit on file sizes for pre-7 FMP files.
When you hit that limit the file will quit. Done, you cannot open it again.
I have done lots of cleanup over the years for maxed-out pre-7 files. The solution when this happens is to have a clean clone of the file that you were clever enough to save before there were any problems (or it's too late!)
Then you take the 2-gig file that won't open, and convert it to a newer version of FMP/A7 or later. Then you figure out what to remove from the data to get the rest back under 2 GB. After removing that, export the remaining data to a different file format than FM so that the Clone of the pre-7 file can read it to import the data.
Don't let it catch you unprepared!