For me it is
CCB sweep attack hitting on 6s. I forget that if you roll a 6 when hitting you can nominate the model to take the save
Arks - i always forget to heal my warriors
Solar pulse - forget to use it until the lascannons open up and then think gee it would be nice to have night fight...oh hang on...
Particle whip is ordnance - oh i didnt pen. wish it was ordnance.
torrent of fire.