Yes it happened to me. I sync with online Ancestry so I take regular backups on an external hard drive.
I can't remember what I was doing at the time but poof! My whole tree was gone. So it has happened to at least one other person.
Oh, yes, I remember one thing. NEVER use File/Close.
File/Exit is OK, closing by the right corner X is OK, double clicking on a different tree is OK, but File/Close does not apparently write the auto-backup file.
Thankfully, restore backup worked and the tree resumed syncing without a problem.