OpUps are very rare, but they do happen.
My wife and I flew CLE-LAS last year on an "elCheapo" when coach was oversold by 1 and F had 1 empty. I was Silver and she had no status.
I had been EUA'd at the 24 hour window and there were some elites (or at least elite companions) on the upgrade waitlist.
I went to the gate, chatted up the GA and voila, she was sitting in F.
Sure that's not BF, but it can happen. Agreed that it's rare though, but it's not zero.
I've only flown to Israel in J/F (twice with miles, once due to a CO.com hiccup) so I've never had the chance to test my luck yet