A friend of mine in Australia has heard from several of her friends that it's unsafe to pour boiling water on car windows in order to defrost them.
Online examples of people claiming it's unsafe include Lifehacker:
Remember to never pour boiling water on your windscreen as the thermal shock from the change in temperatures might result in cracks in the glass.
and this article / press release (not sure which it is) from the Central Western Daily: Hot water may shatter windscreens
THE NRMA has warned Orange motorists to take care when attempting to remove ice from their windscreens.
With the region experiencing night-time temperatures well below zero, many residents have a daily battle dealing with frozen windscreens.
Some people opt to pour hot water over their windscreen, but this could result in cracking or shattering, according to NRMA chief executive Rob Carter.
Mr Carter also warned motorists of the danger of existing chips in the windscreen.
"A seemingly insignificant chip in the glass can cause significant damage if hot or boiling water is poured onto the windscreen when it is covered in a layer of ice.
"In very cold weather, putting the air conditioner on hot can also crack the windscreen if there is a chip in the glass,” Mr Carter said.
To reduce the risk, the NRMA recommends a slow defrost or the use of a plastic card, such as a credit card, to scrape off ice.
You should also lift wipers from the screen. Warm water may be used to remove ice from the windscreen and windows. Never use hot water as it may cause the windscreen to crack.
Is it unsafe to pour hot or boiling water on car windows?