One time at a party my brother grabbed a fire extinguisher and screamed "WOOOO FOG MACHINE!!!!" and almost killed everyone. Everyone in direct "fire" about passed out instantaneously. Lucky there were people outside to run in and drag people out of the house and the doors and windows were all wide open already to air out the cig and weed smoke

Think Halon fire extinguisher.

When it happens naturally in a lake, it is called a Although more to your original question, doesn't need to displace oxygen in the air, it simply needs to be present in the air at sufficient concentrations for your exposure to become fatal. It displaces the oxygen in your blood blood and you eventually suffocate.

methane burps from lakes have been known to suffocate

Yes, heavier than air gasses can be very dangerous.

Many of them don't provoke a buildup response like CO2 does in your lungs so you'll asphyxiate without knowing it.

Large refrigerant cooling systems in enclosed spaces can be a system. I think there have been some guys working on some large systems that died from a gas leak that floods a room.

https://www.hvacschool.org/2017/08/the-deadly-inhalation-risks-that-hvac-mechanics-and-installers-face/

Also seen videos of people throwing dry ice in a pool for the fog effect...Which pools on the surface and people can't get their head through to air.

In an enclosed space, probably.

I got a good laugh out of this. Crazy fuckers.

Yes, but you'd need a LOT of it, and you'd have to do it in an enclosed space. At that point, you might as well just use N2... by the time anyone realizes something is amiss, it's too late; they're already too oxygen deprived to escape or fight back. Using a heavy gas like sulfur hexafluoride would cause instant panic.