After installing Snow Leopard, I noticed some strange behavior, such as having the message pop up at the wrong time as well as having numerous instances of the same message in the message box that pops up to remind me of my event.

I found out that Snow Leopard had made events that I had set to repeat regularly, to repeat only on the day that the new OS was installed, not every other week, or every Friday as I originally set up.

This was easily fixed by changing the events in iCal, but I just wanted to give everyone a heads up to check any repeating events you might have asap.