Here's the deal, everybody:
1) once you are admitted into the FastPass Return queue, you will get on the ride.
2) If you are less than 15 minutes late for your window, it is up to the CM, but you will probably be admitted.
3) If you are more than 15 minutes late for your window, it is up to the CM, but you will probably NOT be admitted.
4) If there is a down-time, your FP will be honored until park closing at that particular attraction.
5)FP staffing depends on crowds, where your FP gets collected depends on staffing. All FastPass attractions have 2 main positions in the FastPass Return queue: Validation and Merge.
Validation checks the time on the tickets and admits guests into the FP Return Queue, Merge collects the tickets at the end of the FP Return queue (where it merges with the Standby queue).
Sometimes, usually later in the evening during off-peak times, there will be no one at Merge, and either Validation takes the tickets, or load takes the tickets. This usually never happens at most FP attractions, however.
The CM at Merge is not, and should not be concerned with times, only with keeping both lines moving as smoothly as possible.