Coming at this from the perspective of both a former instructor and student, here are the best and worst excuses for missing a deadline:
Not every professor will require proof of a visit to the health center, but if she doesn't know you very well (or perhaps too well...), an instructor might ask for evidence of actual illness. That being said, if you are actually suffering from a horrible flu, send an email before class notifying your professor you won't be there, and asking for instructions on how to make up the missed material.
My [family member] died.
I know that some students lie about this one, but I think most instructors will give you the benefit of a doubt here, so don't abuse their trust. If the news was upsetting and you need to miss class, send an email to the professor. If you are going home for the funeral, ask them what you need to do while you are gone. If there is a paper or exam coming up, do not ask for an extension - wait for them to offer it to you. Most will.
I have a job interview.
Again, give prior notice, but in most cases, instructors understand that most people are going to college and graduate school with the objective to find work after they graduate. I never had a problem being excused from class on the days I had interviews. However, you might want to hand in homework or sit for a quiz in advance, since most professors will not accept an interview as a reason for missing assignments or being unprepared for a test.
GIVE IT A SHOT
I'll be really honest - I wouldn't accept this as an excuse. But some instructors might. It's not that I don't believe students have to deal with malfunctioning computers or temperamental printers. I certainly have experience with unreliable technology. But there are so many on-campus resources to turn to if your own computer or printer isn't working. But who knows? Maybe your professor is more magnanimous than I, and will give you a grace period.
I never received that email.
This can be a reasonable excuse since professors sometimes have incomplete listservs or your email sent something to spam. Again, it's a little suspicious if it's a small class, you know people, and there were other ways to be notified of important changes to the syllabus. Still, I've legitimately not received emails from instructors. Just make sure to resolve the issue quickly, since this excuse is not acceptable on an ongoing basis.
I haven't slept in 2 days and am feeling really overwhelmed.
Some people might disagree with me, but I think it's ok for a student to recognize and intervene in their own emotional health with a day off. College and graduate school can be extremely stressful environments. I struggled with insomnia throughout most of my undergrad and master's programs, roaming the halls at 2 a.m. I was desperate to sleep but couldn't shut my mind off.
My first year at Harvard I once went 3 nights without sleeping in a row. I finally got myself to the health clinic, where a counselor took one look at me and announced, "You'll sleep tonight. You look terrible." (She was right on both counts; I was all pale, bag-eyed, and lethargic, and that night I slept 10 glorious hours). Insomnia is something I will always be sympathetic towards, since there is no proven cure for it and can genuinely impair a person's happiness and emotional well-being.
The only catch is that not sleeping means you should be getting MORE work done, not less. Not sleeping is a valid excuse for skipping a class one day, one time, when you really need it.
DON'T EVEN TRY
This is the worst, most cringe-worthy excuse. There's a syllabus for a reason. I know it's super boring when professors go over it on the first day but they are doing it for a reason. Take out a pen or highlighter and circle important dates. Then, when you get a chance, enter deadlines for assignments and tests on a calendar. I usually put things on my Google calendar, and set up advance notifications.
I didn't feel like it.
Um, ok? You're paying how much to be here?
This excuse is particularly grating to the instructor because the students who never seem to feel like coming to class are the same ones who will protest their abysmal grades.
I had a really important paper in my other class, so I didn't have time to do the homework for this class.
Congratulations. You've just told your professor that you lack the necessary skills to juggle multiple obligations, the ability to plan ahead, and - worst of all - that his class is your lowest priority. Don't expect him to be sympathetic.
I turned my paper in, but maybe you lost it?
Doubtful. Sure, there are scatterbrained instructors, but if you're the same student in rarely comes to class or does her homework, they're not going to accept the blame for your missing work. Also, you've just insulted them.
Oh, and check out some truly awful student excuses here. Just don't get any ideas.