I haven't had time to really test this and see if it's an occasional bug or an actual repeatable bug but, I had a Typewriter Text Element which needed to be extended a few seconds after it completed it's action. So, opening it up in the Viewer and hitting Shift+N (for still frame) I noticed that it removed the last one or two frames of it's typing action.
So, instead of it reading Day014 it would suddenly read Day01 when I hit the key combo. I'll investigate it further but it was repeatable that day and I didn't try restarting FCP to see if that cleared it up. The project isn't over yet so I'll have more time to test it later.