After taken apart my PB 301+ I witnessed a sturdy assembly (well, putting it all back was easy and nothing broke - except the glass display/circuit board assembly when I tried prying them apart, based on the Web pictures, but my system was different with the glass glued to the circuit board).
All the buttons went right in place, the case holding tabs snapped OK, not fell off (had happened with other electronic devices).
The glass had initially a little crack starting from an edge: this could be an original glass manufacturing defect (the glass I think goes through thermo-processing, etc.)
It might have developed in a weak spot due to a pressure from a button (mine was close to a button). Or just by itself - being one of a bad lot.
I would not blame the assembly.
A little travel on the Web shows cases like that with different devices: you finished reading at night, wake up in the morning, open the book and see horrible screen streaks. Then you start believing in little green people :-)