Why Did This Glass Break?

Redditor xclever_name_user writes "Pouring a beer last night at work and part of the glass just kinda fell out." He says that the edges aren't sharp at all but "smooth as butter."

I've broken many glasses, but I've never seen one pop out a smooth section without further fracturing. How did this happen?

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Something similar happened to a pane of glass left in our backyard. Every day, a shaft of sun would pierce through the neighbouring buildings, falling only on a specific section of the glass. One day, that section just fell out, cleanly. Perhaps the glass was sitting in a windowsill?

In any case, the general consensus here in the comments seems to be that this is some sort of thermal fracture. Day in and day out, the glass was heated and cooled in a specific pattern, stressing the structure of the glass. Then one day, it just fractured, cleanly.
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Here's a theory. I'd combine the idea of a warmer glass, and a colder beer, but combine it with the beer being poured in roughly the same spot over and over. If you tip that glass over, as if to pour a beer in, that's the approximate splash area.
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Heat from a dishwasher could influence the tempering, however, I cannot imagine a bartender dumb enough to pour cold beer into a hot glass. Burn risk aside, serving tepid beer in a warm glass is a quick way to lose customers. Most likely, the action of the dishwasher jostling glasses against each other in 180-190 degree water would cause additional stress on the temper.
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