Latest I've had this happen is after take-off, and before pushback. Final W&B numbers didn't match, and I wasn't on the flight in the computer (with a BP in my hand, belted to my seat, after having consumed my pre-depart).

I had come in on a late international and barely made it, and apparently I had been rebooked after boarding (yes, UA's systems are that bad). Final check was W&B matching the number of heads the FA counted vs. what was in the computer. Cost us almost an hour before we could take off. UA had to put me back on the flight in the computer while we were waiting in the penalty box.