Glass is a liquid that flows very slowly
The old claim turned out to be wrong and was rejected by evidence.
TaughtGlass is a supercooled liquid. The proof is old cathedral windows, which are thicker at the bottom because the glass has flowed downward over centuries.
NowGlass is an amorphous solid. It does not flow at room temperature on any human or civilizational timescale. Medieval windows are uneven because of how they were made.
What actually happened
The cathedral window evidence is the load-bearing part of the story, and it collapses first. Medieval and early modern window glass was made by the crown method: spinning molten glass into a disc, which comes out thicker toward the rim. Glaziers then cut panes from the disc and, sensibly, installed the heavier edge at the bottom. Some panes were installed the other way up and are thicker at the top, which no flow theory can explain. Roman glass thousands of years older shows no sagging at all.
Edgar Zanotto calculated in 1998 how long window glass would actually take to flow perceptibly at room temperature. The answer exceeded the age of the universe by many orders of magnitude. Glass is a solid whose molecules lack long-range crystalline order, which makes it structurally strange and worth teaching, but the classroom demonstration everyone remembers is false.
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Who was taught this
Still standard through 2010, so anyone who finished school between 1950 and 1998 learned the earlier version.