Most likely the belt tensioner has worn bearings. You'll find that out when the belt slips off again. See if the pulley is sloppy or loose. It's also possible that you have a bad bearing on another smooth surface pulley. Smooth surface pulleys don't have anything that keeps the belt on the pulley. They rely on the grooved pulleys to keep the belt aligned.

You need a new belt tensioner immediately, your current one obviously is not doing it's job if your belt is slipping off , I'd buy a new belt as we'll seeing how old worn serpentine belts sometimes tend to strech, also when you remove the old belt clean all pullies of oil or any lubricant

Check your belt tensioner..

While on pop the hood and check the belt of play.

See if the belt moves out of place and also check the belt tensioner if it moves too much. (it shouldn't)

the belt tensioner is weak, a pulley is bent, oil on the belt ! any of those things can cause the belt to slip off !

That's a very serious situation. Recommend you have it corrected immediately or risk really expensive repairs.

I have a 1998 Ford Escort ZX2 with over 200k miles so I know she's not a spring chicken. Sometimes the serpentine belt slips off the pulleys (it's happened 3 times in the past year) and it has always been after driving in a downpour or through a puddle. I avoid puddles when possible, but I have hit a few and drove in heavy rain & the belt doesn't always slip off, just sometimes. One time it had to be replaced because of missing teeth, but the past 2 times the belt looked new. I have the correct belt, we triple checked. I was wondering if anyone knows anything I can try to avoid belt slippage because it is a pain in the *** waiting to be rescued when it happens. Thanks.