This is a problem i face both in HL2 and HL2 ep1.
On buildings on which you are required to walk on the edges to go to other side, i always fall off. The edge seems to be too low on width! Wonder if anybody faced similar issues?
The problem comes when there is a 90 degree edge, and i have to go around that edge. (i hope you understand what i am trying to say). To go around the edge, i press the right button but it seems to go too much to right and hence fall off!