[ct181]

For me it happened only if I let OmniFocus "open" over midnight (I always put my MacBook to sleep instead of turning it off) and had tasks that are not becoming available exactly at midnight. Restarting OmniFocus was the only solution for me but I think the Omni people already fixed this problem in the vapor trail so it will be fixed with the next update.

Until then, maybe force yourself to quit OmniFocus when you go to bed and reopen it when you wake up.