I routinely set up appointment groups, sometimes involving as many as 30 individual appointments.
The other day, I was trying to add a few extra timeslots to an appointment group, and on the way out of editing the group, my mouse staggered over the "Delete Group" button.
Before I could so much as go "Nooooooooooooo!" the whole group was gone, dusted, dead.
All those appointments had to be manually reconstructed from the confirmation emails.
An action as catastrophic as this, with direct effects on calendaring, should definitely require a confirmation prompt, particularly when the delete button is larger than the other two, and the first of the three options available. This is bad UX.