like others have said, besides double-tapping and making sure it's dead, never, I mean never leave your back exposed towards the apparent "dead" enemy (plus always have a set of eyes over it).
I'm more baffled by the trope that the MC always needs to get some kind of shock therapy from seeing someone else die/dying in front of him before he unleashes his hidden power.
@Invader_Retro Yeah, especially when that something looks dead yet still moving in the first place. Nothing about that thing suggested it would be stopped by damage that would stop a human's vital functions, so it would be foolish to assume so.