Why did my fireplace door glass shatter

[RickH]

Check the inside edge of the frame for a screw or burr that may have stressed it on the small edge.

Tempered glass is more sensitive to strikes/stress from the edge than face on.

I replaced a pane in a commercial window one time, the screws that held the frame together were so close to the glass that the second the screw thread touched the edge as I put it back together, it shattered. I probably could have thrown a baseball at this glass head- on and not broken it, but the smallest concentrated inward pressure at the edge will shatter the whole pane.

[James Sweet]

I've seen it happen with pinball machine playfield glasses. They can take a heck of a smack of the steel ball hitting them from the inside, people smacking their beer down on the outside, etc, but lean one standing on edge against the wall and a grain of sand on the floor can cause it to shatter into tiny shards.

[Norminn]

The windshield on my older car broke twice from a very small point of rust on the frame. This occurred, beginning with a crack that slowly extended, twice within a year. On the second repair, the installer found the cause and corrected it.

[Oren]

The grain of sand comment it true; based on lessons taught to me. Any little nick on the edge of tempered glass may cause the glass to shatter.

A good reason why glass companies use rubber pads during transport to a job.

[hrhofmann]

Got the glass and put it in, making sure there was nothing touching the edges, just on the flat surfaces.