OFFICIALLY, Mayweather has only been down once in his career, in the sixth round of his decision win over Carlos Hernandez in 2001. However, it came from Floyd injuring his hand while landing a punch, rather than from being hit.
Since then, Shane Mosley is the only man to have come close to hurting Mayweather, rocking him with a brace of rights in their 2010 clash. His chin holds up well when clipped, though his ability to roll with punches helps to take the sting out of them.
Berto has been down numerous times in his career, mainly due to his tendency to get involved in firefights.