A university student goes to his professor to complain that his exam grade was added up incorrectly; his total score was too low..
The professor looks at it and says, "Yes, you are right, my teaching assistant added up your exam numbers wrong and gave you too little credit. However, in looking over the records, I see the teaching assistant gave you too much credit on the exam previous to this one. Why didn't you call my attention to his mistake on that occasion?"
"Umm... well, Professor," stammered the student, "I was willing to overlook one error, but when it happened twice, I felt I had to say something."