It's kind of simple really: if you feel like you're going to take a trip to snap city on the final rep and things start to hurt that shouldn't, end the set.
Training log: http://muscleandbrawn.com/forums/showthread.php?t=15881
"No citizen has a right to be an amateur in the matter of physical training…what a disgrace it is for a man to grow old without ever seeing the beauty and strength of which his body is capable." – Socrates
"I never saved anything for the swim back."
"Compound exercises, kids! Compound exercises!"