Sheared glass

I poured some wine and thought there was something strange about the proportions of the glass as I put it to my lips. Although the rim wasn't sharp I realised almost immediately that something was wrong. The rim was cut rather than rolled. Putting it next to a comparison glass, it was clear that a piece about 1cm wide had disappeared.

It was remarkable that the break was so even. Investigation in the dishwasher yielded a perfect ring. Speaking to my sister on the telephone later revealed that this isn't a unique occurence. How does it happen?