I found this bug while playing portal ,it can help you on challenge missions
it is not very hard ,you just have to try it some times

video:
http://www.youtube.com/watch?v=KI_v42hNUYI

Hehe

Didn't saw that before.

Yeah, I've seen it before; there is even a place in my latest map where I had to move one of my cameras because I was jumping down from a higher ledge to start a timed series of jumps. . .and would sometimes accidentally land on the camera below (and the delay would make me miss my goal).

cool thing will try it by myself. thanks