So remember how Ten got his hand cut off in The Christmas Invasion? And then he grew a new hand?
Well, that hand is slightly newer than the rest of his body, so when he regenerated, that hand still had some of the Doctor’s Tenth regeneration in it, and occasionally it reminds the Eleventh Doctor of that fact. It does this by doing things like letting Eleven know when he’s being stupid.
– kevinfreaknsolo on Tumblr