What are the worst mistakes in doubles in your opinion?

[beernutz]

My top ones are (from worst to less worse):

1. double faulting
1. hitting an unforced error on second serve returns
(they are tied for #1)
3. not getting a high percentage of first serves in
4. not following the deep-to-deep, short-to-short principle (e.g., overusing the low percentage shot down the alley around the up man)
5. lack of communication between partners (e.g. no verbal help at all on switching, calling 'out' balls, deciding on a strategy for middle balls, etc)
6. guarding/hugging the line when your partner is serving
7. expecting your partner to cover all lobs
8. failure to put away either volley or overhead sitters (the worst case of this is actually committing an UE on a sitter)

Those are mine but I'd be interested in others.