I'd guess most here have seen the military's improvised booby trap using 12 gauge shells, a rat trap, and a trip wire.
I've seen those when on emergency runs
When in training I got to do ride along with big city/county service. They wore kevlar and in many areas LE went in first, because fire and medical are just considered moving targets. After one run we were taken back to the scene to see all the danger we faced if we hadn't allowed others to clear the way.