I have to disagree with this. I know it is very unusual since the ignition point is around 500 degrees. But i was told by some herper friends who had a shop that an UTH they sold to a customer had overheated and caused the glass to crack.
I did read just the other day, in "The Complete Ball Python" (everyone knows that book,, that UTH can and will crack glass if they are not regulated and the hot spot suddenly gets cooled down. Water spilled on that area?
So yes it possible. Not very likely. But it has happened.