I watched a news story about 3 volunteer firemen who followed each other into a liquid manure tank once.
The farmer had stuck his head into the inspection hatch, got hit by the H2S gas and fell in. (not sure what he was trying to do there, something about the pump I think.)

His son tried to reach in and drag him out, 2 down.
The second son called the fire department and they did the same thing as son number one, one after the other.
One tank, 5 dead.

Most people just don't understand the hazard of confined spaces and if they see somebody go down their first instinct is to reach them and try to help them.