Just sometimes a scene will stick. It kinda judders to a halt for no good reason, well, not one you can see at the time.
You're happily writing away, you read the scene, it makes sense, you're joyful and move on.
Then one day that scene comes back and kicks you, really hard in the shins and demands that you fix it.
You read it and it all makes sense. You tell it to STFU and move on.
It won't go quietly.
Next thing you know it smacks you upside the head and demands you listen.
You read it again.
You read it out loud and backwards and all of a sudden you see it ... or rather you don't see it. That one word that you omitted quite by accident.
The difference between your main character standing right and front of someone while stabbing them in the back or being where you thought they were, behind the person jamming that knife between their ribs.
Read your work.
Read it carefully.
Read it out loud and backwards and keep reading it until you find the thing that's making the scne smack you around. :)
Also, it doesn't hurt to grab someone (or a few someones, depending on how much taller or shorter etc your main character is than you) about the same build and height of the person your main character is fighting and move through the fight as you describe it ... trust me. It helps to get off your ass and out of that chair and play a bit!
And now for something completely different: