This sounds to me like a corrupt file with the OS incapable of detecting the end of file marker. You may be able to resolve this by cutting the old document, bit by bit into a new document and testing at each stage until the problem re-curs. In fasct oyu could find trhat simply cutting the entire cocument into a new one will clear the problem.
As for what causes this sort of error. Who knows, but I would put my money on a keying error at some stage where you typed and unreadable character by pressing too many keys at the same time.