This has happened to me three times on my 622, the most recent being last week. Each time, it's happened to very few or no other people, at least amongst users here. Other times it's happened to other people but not to me.
You can sometimes force it to clean itself up by rebooting the receiver, then running the "check switch" routine in the setup menu. But it may still take a few hours to clean out the bogus timer events after the guide repopulates. In fact, this last time it happened, I'm still getting timer events for reruns when they're supposed to be for new shows only, on shows that have full descriptions, episode numbers, and original air dates.