I was just sitting here at my desk. Next to me is a plain tall glass that I recently finished drinking a lukewarm ice tea drink from. It was empty. I heard a *ping* and I noticed a crack around the bottom of it, just above the base. I picked it up and the bottom came off.
Never had that happen before. Glass spontaneously breaking.
A quick internet search shows that this happens sometimes to all sorts of glass. Resonance frequencies, or imperceptable stresses to the glass.
Time to get a new glass. Maybe a plastic one this time.