Glacial = anhydrous. People call it this way because pure acetic acid has a rather high freezing point (16°C), and it looks a lot like plain ice when frozen. (It still stinks vinegar once you open the bottle though.) But once you add even a bit of water, the freezing point drops considerably, so acetic acid solutions don't show the same "ice".

So in colder days, you need to rewarm it back into a liquid. Then people get really sloppy (I know it not just from that professor's anecdote, but from watching it). They say "I'm just rewarming it, and it's just acetic acid, what could go wrong?". Well, it's still a big flask of a corrosive, volatile, and flammable substance.

In the meantime, the same people doing dangerous reactions like nitration (it literally explodes if you let it get too hot - spreading nitric acid, sulphuric acid, and some carcinogenic solvent) "miraculously" pay full attention, obsessively taking care of the temperature of the ice bath.

Part of the advice that I mentioned in that comment chain is that - smaller dangers are still dangers, do not underestimate them.