I successfully finished a 'Fix the Defense Grid Computer' mission, the computer disappeared, the mission icon disappeared from my mission menu and the mission key went poof, too.

I never got the music to let me know the mission completed, I never received the reward item (Phantom Weight nano crystal for my friend who helped me complete the mission) nor the money or experience.

Coupled with the numerous reports of Item-Watching missions not working, it makes me wonder what mission types actually do work (did an assasination mission, it does work)... :-(