Quoted from TigerLaw:

The glass will sometimes just inexplicably shatter. This happened to Pinside moderator Marcus not too long ago (maybe three months ago). I've heard of it happening to others as well. Just a temparture change can sometimes trigger it.

Quoted from ForceFlow:

Actually, that's a myth.
Glass is as solid as any other solid.
The deformities in antique glass simply have to do with the old imperfect methods used to form the glass in the first place.
http://io9.gizmodo.com/the-glass-is-a-liquid-myth-has-finally-been-destroyed-496190894

Quoted from ForceFlow:

From the article:
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Whatever flow glass manages, however, does not explain why some antique windows are thicker at the bottom. Other, even older glasses do not share the same melted look. In fact, ancient Egyptian vessels have none of this sagging, says Robert Brill, an antique glass researcher at the Corning Museum of Glass in Corning, N.Y. Furthermore, cathedral glass should not flow because it is hundreds of degrees below its glass-transition temperature, Ediger adds. A mathematical model shows it would take longer than the universe has existed for room temperature cathedral glass to rearrange itself to appear melted.
Why old European glass is thicker at one end probably depends on how the glass was made. At that time, glassblowers created glass cylinders that were then flattened to make panes of glass. The resulting pieces may never have been uniformly flat and workers installing the windows preferred, for one reason or another, to put the thicker sides of the pane at the bottom. This gives them a melted look, but does not mean glass is a true liquid.
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