I’ve found a bug in unforseen consequences at the section with the bridge that gets destroyed by the exploding crates, straight after the elevator with the headcrab section. There is a scripted event whereby a houndeye and a headcrab attack a guard, blowing the three crates which thereby destroys the bridge and forces you to go over the pipes.
However, if you kill the houndeye the crates do not blow. An invisible wall prevents you from crossing the bridge. Loading into the next section reveals that the bridge is now blown and the crates remain in place hovering in the air. Blowing the crates causes the bridge to reappear, collapse, then reappear and go through the collapse animation again. I am not sure how this bug has not been spotted yet, but considering the amount of times in the game whereby Gordon can save characters during scripted events, I think this needs looking at.