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?
Every bar and pub I know,hand-washes their mugs, pilsners and pints in a sink behind the bar.
Bar glassware takes a considerable beating even when the patrons are behaving.
On slow nights we’d make a few bucks on bar bets that we could drop a glass in the floor and catch it before it broke.
Those 9 oz water glasses with the wavy sides you see in diners and chain pancake houses will bounce on a hard floor if dropped correctly.
We’d drop one from about waist height on the tile floor, and it would bounce twice. If it hit the floor a third time it would shatter.
One customer explained how it worked. Apparently, the tempered glasses would absorb the energy of impacts. Eventually. the glass could take no more and even the slightest tap would cause it to shatter.
I’m not sure, but I suspect the same principle applies to bar glass stress as it applies to Prince Rupert’s drops.
http://www.popsci.com/diy/article/2008-06/shattering-strongest-glass
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.