I wont give anything away about the story here, just the glitch.
I'm about 62 missions through the game (or so). I started playing through the mission, and then in the building, suddenly all the furniture dissapeared, and the disc refused to load any new objects. I was forced to find invisible enemies and kill them, thanks to the auto aim, had to go down invisible stairs and had to find my way around objects (e.g. desks or cabinets etc) in the level. I had to walk out the door at the end, it was all blue and invisible enemies again - and some of the invisible cars parked outside exploded nearly killing me.
Luckily, the disc began loading thing in again - it was just an annoying thing to happen.