That seems to happen more often than people would be led to believe. My parents were on the 747 (to SIN) in C and after boarding found that two other pax had taken their seats - turns out that LH had allocated the exact same seats to them. Strange how the check-in system could allow for that, and how no red flags came up when BPs are processed upon boarding...

Anyway, the lady in charge onboard swiftly took action, and as C was completely full... she invited my parents to the upper deck.

So the moral of the story is: even if you don't get an op-up at the check-in desk, even if you don't get op-up'd upon boarding... you might get lucky and get an impromptu upgrade once you're on the plane.

Or for the less fortunate, as Weero's example shows, you might get removed from the plane at the very last instant.