For me, the things that trouble me the most are positioning errors.
We all can only hit the shots we can hit. So I can live with pitiful serves, botched volleys, lame lobs.
I do go a little nuts with partners who alley camp. Equally awful are partners who stand in the same spot for an entire set.
Positioning doesn't take athleticism; all you need is awareness.
-- Random Error Generator, Version 4.0
-- Master Moonballer