Thermal Shock

Thermal shock is a term for a break caused by a too rapid change of temperature within a piece of glass.

"Glass tends to be 

1) very brittle, 

2) expand and contract quickly when subjected to temperature changes, and 

3) is an insulator (when solid) and therefore does not readily conduct heat. 
That is why glass is highly susceptible to thermal shock"

http://www.glassfacts.info/indexf286.html?fid=210

This can occur on both an increase or decrease in temperature. Glass conducts heat poorly.  The ideal is to keep the temperature differentials within the glass to 5C or less.  This is the purpose of the anneal cool.  The risk of thermal shock can be increased by different thicknesses across the piece. Greater care is required in cooling these pieces than those of uniform thickness.

A piece showing large differences in thickness and so at greater risk of shock

Identification

The break normally is straight through the glass without following the edges of the various pieces of glass.

This shows the break crossing multiple colours of glass

The line of the break will be rounded if it parted on the heat up. In some cases, the glass will have stuck back together if it was dammed or the break was gentle enough to avoid pushing the glass apart.

If the shock occurred on the cool down, the edges will be sharp.  

The edges will also be sharp in a slump whether the break occurred on the advance or the reduction in temperature.  If the pieces fit together perfectly the break is likely to be in the down phase.  If the pieces are slightly different shapes the break likely occurred in the rise in temperature phase.

Other kinds of breaks are possible and are described elsewhere.