Huh.

I decided, on a whim, that I wanted spaghetti for supper--well, macaroni with spaghetti sauce.  I made the sauce I grew up on, and to save time later, decided to put most of the sauce away before I ate.  I rinsed a glass jar out with hot water (to prevent cold jar from breaking when hot sauce was ladled in), and rinsed the lid with the same, since it'd been sitting in its box since I got it and hadn't used it.  I put the sauce in the jar, wiped down the rim, and put the lid on.  Then, I set it on the counter to cool, so that it wouldn't make a mess in my fridge.

I went to go check to see if it was cool enough to put in the fridge a few minutes ago, and found, to my surprise, that the damn thing sealed itself. 

I have accidentally canned* spaghetti sauce.

*Since the jar and lid have not been sterilized, I'll be putting it in the fridge anyway, and using it pretty quickly.