Some of my appointments disappear and I can't figure out why.

A handful of appointments don't appear in the "normal" day/week/month view
of the calendar. But if I change to List View, they're there (View Current
View All Appointments).

If I edit the appointment slightly (such as adding a note or changing the
category flag), the appointment will appear in "normal" day/week/month view,
but only temporarily. If I scroll ahead to the next month and then scroll
back, the appointment's gone again.

I can't find any way to make these mystery appointments "stick."

I have no filtering on. I'm using standard Office views, nothing custom. I
have only one calendar named "Calendar" in my Personal Folders, plus the
Shared Calendar. Any ideas?