It's said that we all learn from our own mistakes, but the beginning of wisdom is learning from those of others.
The website LearnFromMyFail.failblog.org helps you to do this, or at least have fun reading about other people's screwups. For example:
When you have a cold and your nose is sore from blowing it, check that the tube in your medicine cabinet is, in fact, ultra-moisturizing gel. Wart remover on sore chapped skin will bring a grown man to his knees.
When your friend is telling you about his new flat with two female flat mates, it's probably better to wait until your girlfriend is out of the room before asking "are they hot?"
Before reporting to the librarian that the copier is malfunctioning by printing everything extra light and in mirror image, look at the 'backs' of the pages to see if they come out of the copier upside down.
For the love of all that is holy, re-read your post before you hit "submit," otherwise your parents, RA, and a counselor will be waiting in your dorm when you return from MATH lab for a METH intervention.
Don't ask your parents what they did for Valentine's Day.
If you're on the street and think your makeup looks messed up, just find a mirror and check it. Don't try to use a parked car's window, there might be a person inside the car looking back at you.
When panicking about the location of your glasses, stop for a moment and think about why you can see anything.
When waving your hand over a bunsen burner in science class to impress your friends, don't.
DO NOT think that cramming pancakes in your mouth at the last second will stop you from sneezing.
Incidentally, before you send a helpful message to the Learn from My Fail people to tell them the noun they're looking for is "failure," note that the same folks have a site called "ICanHasCheezburger.com." Pick your battles.