Knowing the correct moves in joseki is useful, but often more useful is knowing how to punish incorrect moves.
(Many thanks to all who have commented, please keep those variations coming!)
C[This is a common joseki, occuring when white invades the three-three point in response to a black pincer. However the last black move is a mistake. Do you know how to punish it?]
C[The four black stones are captured]
C[a and b are miai]