Our first trip, I was a bit overwhelmed by all the information I had read here, and in guide books, and everywhere else, and by the shear size and complexity of Disney World, and it completely skipped my mind that I had ready about using fast passes beyond their return window, so we spent a good deal of energy trying to get to the rides during the return window, until one day, toward the end of the trip, when we returned to Test Track and it was broken down. The CM at the entrance took the time to explain to me that we could use our fast pass, or any other fast pass, at any time during that day as long as it was past the start time for the pass. As she was explaining it, the light went on in my brain that I had read that here, and in UOG, and for the remainder of the trip, and the 11 days we spent in the parks last summer, we didn't worry about the return windows.
We didn't have any problems at any time, including using a BTMRR fast pass from 10:00 in the morning during EMH that night. This, of course, is a hotly debated topic here on the DIS, and is sure to draw some fire, but the important fact is that Disney doesn't stop you from using the fast passes late, so use em when you want, and enjoy.