AP Exams are standardized tests that are given at the end of a school year to high-school students that have studied material at a college level. The first part of an AP Exam is the multiple choice section, which is completed and sent to a place where it is graded by a machine.
All of these tests recommend (or mandate) that the test-taker use a wooden number two pencil and NOT a mechanical pencil when taking the multiple choice portion. For example, from the site describing procedures for the SAT:
Here’s what you can do to make sure your answer sheet is scanned accurately: (...) Use a No. 2 pencil and a soft eraser. Do not use a pen or mechanical pencil.
A large number of students I know and I have used mechanical pencils on standardized tests including the above exams and have never had problems with our tests being graded (I used exclusively mechanical pencils on all of the above tests and have never had an issue in the score).
Are there any instances where a student's use of a mechanical pencil caused a test to be scored incorrectly by a machine? Additionally, what is different between a wooden, number two pencil and a mechanical pencil that justifies the restriction of mechanical pencils?