I saw this happen to my neighbour just the other day - warm water on windscreen, windscreen shattered.
".... hot water brings a real risk of damaging a screen. Sudden expansion caused by hot water followed by contraction as it cools can cause the glass to crack, especially if it already has chips or small cracks.
On very cold days this method is ineffective, as the hot water will rapidly cool and almost instantly turn to ice. Worse, it will accumulate on the wipers and freeze them to the screen, risking damage to the wiper motor or ripping off the rubber if you switch them on."
http://www.theaa.com/newsroom/news-2012/defrosting-your-car-aa-autowindshields-advice.html
The run off water also creates a skid pad to help the postman reach your letterbox that bit quicker.