admittedly extreme devil's advocacy:  let's say the coach gave the jersey to  what appeared to be a 26-year old woman.  or a mom from the crowd.  or a 
man.  (I'm picturing Coolidge from the White Shadow squeezing into that #21 shirt.

)
It's still not our call, right?  We just have the book changed and assess the T.  If the other coach wants to file a protest with the league, that's his/her business.
Because we're never in the business of verifying school IDs, driver's licenses, or--gulp--primary sex characteristics, right?...if it did come to that, though, whose purview would include the locker room peek?  The R or the U?