Interesting thread. I'll bite, though I don't think anyone should make excuses for Floyd if he loses.

He was too small as a welter (though that would ignore Mosley's progression from LW).

He was rusty. The JMM fight was such a gimme that he hadn't had a challenge in over 2 years.

Mosley is the first legit puncher he has faced above JWW.

He had a bad chin that was hid by his excellent defense. Mosley just caught him with a lucky/well-timed shot.

He got old and past his prime over night.

His socks were itchy.

Not really an excuse, but people will say that's why he's been ducking a legit WW.