I once spied on people playing Spider-Man 2 at a demo kiosk. What they most wanted to do was to see if they could kill Spider-Man by jumping from a high enough place. Eventually they succeeded, got a good laugh out of it, and went about their business. Funny, nobody did that in our focus tests.
Likewise, something I've devoted a fair amount of energy to while playing Fable and The Sims 2 is getting my characters laid. Would I do that in a focus test? Probably not. Not even if the observer was behind a mirror.
Focus testing is a good tool for finding shelf-level events. For measuring 'fun'...not so much.