This is simple and kind of lame, but I've always wondered, seriously
They use Halon protection in a lot of computer rooms and datacenters. They have alarms on them and you're warned to get out (beforehand) if they ever go off. Apparently it will kill you if you're inside when it goes off, because it robs the air of oxygen to put out the fire. The warning signs they have to put on them are pretty serious-sounding.
Now, if you're stuck in the room when one of these things goes off, are you a definitely a goner? Will it kill you instantly? If it's supposed to, has anyone ever survived? Or will it just knock you the f*** out??
If it goes off would you have time to get to the door before it's too late??