Not entirely sure what happened here. I was in the 19th test chamber, I paused the game to go do something, came back and fired off a Blue Portal. All of a sudden I am falling through the map.

I reloaded my save, because I never died...just kept falling. And it seemed that whenever I fired a Blue Portal on the opposite side of the map I fell through the floor. Orange Portals were ok anywhere.

*shrug*

Enjoy!

EDIT: Sorry for the low grade video quality, no capture card here!