I've had two windows break in 737 cockpits. The second one, I watched happen:
It started with arcing in the internal heating element within the window. It looked like a tissue which has been touched on the other side by a lit cigarette - an expanding ring of bright red, crawling outwards. I turned off the window heat for that window and it immediately stopped, however the heat that had already been produced turned the centre of the burn area a milky white as the heat affected the Perspex. A small blister then formed and the glass layer almost immediately shattered with a bang.