I've discovered that the error is caused by my form being regarded as a "One
- off". This actually occurs when I add a userproperty to the item. You can
tell this has happened because the item.MessageClass changes from
"item.appointment.myform" to "item.appointmentItem". So you can't add
userproperties.

I still need to find a way to mark the item temporarily as having been
drag-dropped so my item_Open code can perform a different action from the
normal open. Any ideas how I can do this?