> when I was doing A-level chemistry at school circa 1970 we were told always to check that any glassware we were heating up a lot (or might be very exothermic) had the PYREX symbol on it, otherwise bad things could happen

Speaking of "very exothermic" ... decades ago a lab colleague of mine once entirely blew out both (glass) sides of his fume cupboard while attempting to melt "ice" with a heat gun. Of course the "ice" that had formed in his flask during the previous reaction turned out not to be ice. The flask itself was later found in lots of very very tiny pieces.

It was the loudest bang I've ever heard, happily he sustained only very minor injuries, with hindsight a minor miracle.

There should really be a reality show about chemists.