I went to the dentist to get several cavities filled. The dentist's usual assistant was sick for the day, so the secretary was filling in. (Evidently she had the credentials too, but was a little out of practice.) Everyone was off their stride. The dentist was in my mouth for a while. Finally, he was done, and he handed me a mirror so I could take a look at my restored mouth.
"What's that hole there for?" I asked, pointing.
"Ooops," he said, and cranked the chair back again, and put in the filling.