When you feel you have given it all, or maybe you just feel lazy that day, know that, deep down, you know you can. That extra rep, or half rep is there. Of course, sometimes it can be dangerous, like with a barbell over your chest. The rest of the time, however, the damage of trying is far less.
You just might be surprised with what you can do with the last kick or the Hail Mary pass as time expires. As long as the clock is ticking, why should being tired be an excuse?