At the start of the exam, the invigilator says, "You have exactly 2 hours. I will not accept any papers after this time has elapsed."
Two hours later, the invigilator calls out, "Time's up, Ladies and Gentlemen."
One student is still scribbling away ten minutes later when the invigilator has all the collected papers in a large stack front of him. Slowly, the student finishes up and walks over to hand in his paper, but the invigilator refuses to accept it. The student puffs up his chest and says:
"Do you have any idea who I am?"
"No," says the invigilator.
"Great," says the student as he slips his paper into the middle of the stack.