At the very beginning when the torso falls on you out of ventilation pipe. You can put a box under the pipe. Then you put the barrel on top of the box and get into the pipe. Inside there is a hole in the world that shows as a white portal to oblivion.
This is a small bug, but still looks unclean.
Already reported, but thanks anyways for posting this.
Founded in 2004, Leakfree.org became one of the first online communities dedicated to Valve’s Source engine development. It is more famously known for the formation of Black Mesa: Source under the 'Leakfree Modification Team' handle in September 2004.