Half-Life: Opposing Force Easter Egg - A Return to the Original HL Training Ground

The transporter weapon has two modes of fire, the primary fires a transporter orb that "gets rid" of enemies, the secondary will transport you to some random safety zone on Zen (this conveniently use's 60% of your energy so it cannot be done again while in Zen). However, when you first get the weapon you are in Zen. If you have enough energy you CAN use the secondary fire there, but instead of taking you to some safe-zone on Zen, you are transported to the training course from the original Half-Life!
When you first get there do not move and watch how the slave reacts to the hologram guide. When you are about to exit the area look at the billboard on the wall, they were not there in the original HL and they also contain a cartoon of game designer Randy Pitchford.