Usher brought out a ton of special guests for his Super Bowl Halftime show -- most of whom were expected -- but one person he didn't trot out ... Justin Bieber, who was in town.
The legendary R&B singer took the stage at Allegiant Stadium Sunday for the Super Bowl Halftime Show -- and he put on a medley for the ages ... going through all his biggest hits, including 'Caught Up,' U Don't Have to Call,' 'Superstar' ... and eventually, 'My Boo.'
Alicia Keys came out and started singing 'If I Ain't Got You' on the piano .