Tempered glass can be temperamental.

I've personally hit an old side window with a large rock and it just bounced off...

but, I was installing one of those after market springs on a rear window wiper on my old Mazda MPV.
It slipped - and the pointy end of the bent spring hit the edge of the rear window.
POW, I was stunned to see the whole rear window shattered into 1/4" pieces. = $1000.00
