Time travelers, beware! Pre-existing conditions are a temporal trap.
Don't get caught off guard by the paradoxes of time.
Try to get a haircut from the barber who gives haircuts to all the other people in town, but only to those who do not already have hair.
Don't try to kill your grandfather before he has children, or you might just end up as a historical footnote.
Be careful not to use a time machine that relies on a bootstrap paradox, or you might just end up creating a closed timelike curve.