I did something similar with a glass plasma globe that I had sitting on top of my MB.
Unsecured, fell during play.
The globe went poof, didn't break the playfield glass, but the globe basically turned into dust.
I've got a bunch of it in the PF rail channels that needs to come out still. It's going to suck.
I'd almost rather deal with shattered tempered glass.
I learned my lesson, that's for sure.
-Cyan