It seems to me that if you have a fastpass that, say, has a return time of 1pm and expires at 2pm, that you are not making the standby line guests wait any longer if you return after the expiration, even if you return at 8pm or something. Because, since you did not show up in the fastpass line between 1 and 2, the standby line people actually waited less time then they were supposed to during that hour, and then that made the standby people between 2 and 3pm wait less time, and then the standby people between 3 and 4pm wait less time, etc etc, until you show up and get in line, and then that actually corrects the day's line back to where it was supposed to be, making all the standby line people after you wait until their proper time, not more than they are supposed to (meaning they would have to wait until the time they would have waited had you gone on earlier anyway). This assumes, of course, a constant line throughout the day, with no periods where there wasn't a line, or breakdowns, or whatnot. Maybe there's a problem with my reasoning here, but I don't see a problem with returning later in the day with a fastpass.