Excuses for being late are rarely successful.
A tired excuse: The night before one exam, two students tied one on, (well, actually, tied two on, one each), and managed to sleep through the final. They realized they were in serious trouble, so they agreed to tell the professor that they had a flat tire on the way to the exam. ``No problem." said the Professor, ``Come by my office at 5 P.M. and I'll give you the exam then." Feeling pretty clever, the students spent the intervening time getting information on the exam from students who had already taken it, and making sure they knew how to do the problems. Coming to the professor's office that evening, they were told, ``Leave your books in my office, and I'll put you in two separate rooms for the exam." They were both ecstatic to see that the Professor had given them the exact same exam taken by the class that morning. However, there was an additional page tacked on the end, upon which was written, ``For 50% of the grade, which tire was flat?"
Make sure you are solving the question that was asked. Many a clever student has given a beautiful and elegant answer to the wrong question because of a careless misreading of the problem .
A far too common mistake is to tackle the problems in the order in which they are given. This leads students to waste disproportionate amounts of time on the first problem they get stuck on. Sometimes this occurs on the first page of the exam, a recipe for disaster.
Never, never, never (that's right, never) leave a problem blank. Professors want to give you points. It is an compulsive human instinct to want to give at least a few points. You leave the problem blank, and they can't give you even a single lousy point. The professor's hands are tied, literally tied. Of course, if you are putting things down, it is definitely preferable to put something down that is relevant to the problem. If it is a word problem, draw a picture that shows you understand what is going on. Label the picture with some variables. If it's a graphing problem, draw some axes, label them and plot a few points. Whatever, just don't ever (that's right, ever) leave a problem blank.
Never panic at the last moment and erase a whole page of work in the last five seconds. If you have to panic, just cross out the wrong stuff with a single line. Tender hearted professors have been known to ignore the line if the work is correct, at least giving partial credit.
Never, never, never (that's right, never) leave an exam early. Use any extra time to go over the exam. On scrap paper, do each problem over from start to finish. Then compare your new answer with your old answer. If there is a discrepancy, figure out which one is right, and the sooner the better. Also check if your answer seems reasonable. If the problem asked for the number of dogs neutered in Walla Walla in 1993, the answer shouldn't be -4,596. As dogs are only too aware, you can't un-neuter a dog.
The only caveat to the ``Don't leave early rule" is if your professor was emotionally impacted by the '60's and gives exams with no time limit. Then when it starts to get dark, and it's only you and the professor left in the exam room, go ahead, pack up your scrap paper and turn in the exam.
In a class of 100 students there is a high probability that at least one person has been robbed on some problem. After grading 45 's, a y'(x) can look very strange and be marked wrong. So if you don't understand what you did incorrectly on a problem, by all means ask the professor or TA. Even if it has been graded correctly, you want to understand your mistakes, so that you don't make them again. Don't bother with those classics, ``I feel I deserve extra points for this problem because I knew how to do it but ran out of time," or ``I was thinking the right answer but wrote it down wrong." Only a very inexperienced teacher would fall for this stuff. Others may even scrutinize your exam for other points to deduct. Professors dislike it when students ask for unjustified extra points on problems. Do so at your own risk.
Your professor is unlikely to improve your grade because you say your parents will hold up payments on your Beamer if you fail the class. Now it's true that begging might work to a limited extent with some inexperienced or exceptionally soft instructors. But even if it did work, you would always know deep inside that those points weren't really deserved, and eventually the guilt would send you crawling through the door of every sleazy bar you could find until, one day, you would wake up, dirty, disheveled and distraught, with a pounding head and a tongue of sandpaper, and you would realize you had to go over to the math building and find that professor and say, ``Please, take those undeserved points away. I can't stand to have them anymore." And the professor would say, ``I don't know you. No one as completely decrepit as you has ever been a student of mine." And the professor would call security and they would throw you back into the nearest gutter off campus. So save yourself the embarrassment.