This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
From: Tom Reingold <email@example.com>
Charles E Newman writes:
$ Something really weird happened at 12:11 AM. My bike blew a
$ tire while just sitting parked in my room. I was awakened by a noise
$ that scared the livin ^&$% out of me. I ran in and found that all the
$ air was rushing out of my tire. How could something like happen in the
$ middle of the night when the bike isn't even being ridden? I have
$ heard of it happening when the bike is being ridden but not when it is
This happened because a bit of your inner tube was pinched between your
tire bead and your rim. Sometimes it takes a while for the inner tube
to creap out from under the tire. Once it does that, it has nothing to
keep the air pressure in, so it blows out. Yes, it's scary. I've had
it happen in the room where I was sleeping.
To prevent this, inflate the tire to about 20 psi and move the tire
left and right, making sure no part of the inner tube is pinched.