Execution errors aren't the worst mistakes in doubles. Even the best players in the world can miss.
What is unforgivable are the positioning/tactical errors. You don't need Mad Racket Skillz to position correctly. You just have to pay attention. Worst Mistakes include:
Failing to switch when a ball goes over your head, and then crouching down in I formation in front of your partner instead.
Expecting a ball to go long and failing to set up to hit it in case it drops in.
Hitting an awesome lob over the net player that touches off a mad scramble but failing to take the net.
Failing to cross when your partner poaches and instead just standing there watching.
Failing to cross on a planned poach and just standing there watching.
Failing to come to net when your partner is lining up an overhead.
-- Random Error Generator, Version 4.0
-- Master Moonballer