hmmm, put it this way! what about next time when you recieve your bank statement and they put a -ve in front of your account balance. :) you know what difference that -ve sign will make. :p
Tests are there to prepare us for real challenges in life, they test our knowledge and improve us in applying that knowledge.
Marks just count on mark sheets. If you have managed to get some knowledge or have discovered mistakes in the test even if there is not the desired grade, you have achieved what was required from the test.