This prank was played on me.
I once worked on cryogenic systems. In my work, I proof pressure tested many parts. Normally high pressure proof testing is done with water (hydraulics), but the parts I worked with could not be exposed to any contaminants including water. So proof pressure testing was done with nitrogen. This was extremely dangerous because compressed gas can be explosive and also can launch parts at speeds similar to bullets. In addition to that danger, there was an asphyxiation danger (That is where an odorless gas can fill a space and push out oxygen causing a person to suffocate without warning).
The room I worked in had a massive maze of high pressure gas pipes all over the walls. There were check valves, manual valves, automatic valves, filters, regulators, gauges, emergency systems, etc. The room was tiny and just big enough to have a desk and some chairs. It had an adjoining room the size of a closet. The wall between the two rooms was steel reinforced concrete with a double walled steel blast door. The door rolled on a track, but was so heavy I couldn't move it without putting my foot on the wall to gain enough leverage to move the door. The back wall of the small room was brick but wasn't reinforced. The opposite side of the wall was the outside of the building which had a fence around the area outside of the wall. The closet sized blast room was designed that way in the event of a catastrophic event. How it would work is the blast wall and the blast door was designed to prevent the shock of the blast or parts from penetrating into the work room while the outside wall was designed to fail allowing the pressure of an explosion to escape. The fence outside prevented people from getting too close to the danger area.
You can see the seriousness of the area I worked in. Despite the efforts built into the blast room, the plumbing, instrumentation, valves, and emergency systems were all housed in the room I worked in and completely covered the walls. I was essentially surrounded by a maze of extremely high pressure plumbing.
One day I was performing a proof pressure test and just finished connecting a part to a high pressure line inside the blast room. I exited the blast room and put my foot against the wall to roll the blast door into place. After verifying my gauges and checking the procedure, I started turning valves and adjusting supply pressure from the maze of high pressure gas plumbing on the wall. What I didn't notice is that my coworker snuck into the work room while I was inside the blast room connecting the part. He then hid under the desk and waited for me. This coworker waited until I got the part almost fully pressurized (many thousands of pounds per square inch). He then started making a hissing sound. Hearing that sound, I went into panic mode trying to figure out what went wrong and if my life was in danger. Soon after, I hear giggling from under the desk. I turn around, look at the desk and out pops my coworker laughing away.
