When you click on Delete: [all slots] and [all unused slots] buttons a warning message pops-up saying: "Deletion is definitive. Continue anyway?" which lets you cancel if you didn't mean to delete everything.

This should also occur for [my unused slots] and [all my slots].

One of our Moodle users recently accidentally deleted all his slots, including all the data connected to each student he'd seen and all the notes he'd attached to each scheduled meeting.

The reason this occurred is that it's not clear what these delete slot buttons do, as they don't all appear under the Delete heading and don't have the word "delete" in the button text. It would be a good idea to name these buttons with Delete before them. I.e.
-Delete all slots
-Delete all unused slots
-Delete my unused slots
-Delete all my slots

This can be done in the Moodle site admin language customisation as a work-around.