“Doors and corners” as Detective Miller was fond of saying.
The button side of jackets thing is true though, except usually also told wrong. We're often taught in fencing that men's jackets button left over right so that a sword drawn by a right handed person doesn't fuck up your own clothing. That's also wrong, but close: men's jackets button left over right because you traditionally put your shielded side forward, your left side, and you wanted any armor pieces to overlap so that there's no holes to get caught in if you're poked from the left. Which is a stance backwards from later style one handed fencing. The not messing up clothes on the draw of a sword is just extra bonus from the original reason.
Going up, your head is exposed, so that becomes the primary target, when people typically aim for center mass. But center mass isn't visible immediately, so head takes all of it. On top of that, the whole 360 degrees of the next floor becomes visible at once, so there's no way to take slices of the room at a time, like you would a doorway.
Staircases are very brutal. But really, room clearing as a whole is the last thing you want to be doing. You can do it completely correct, and still die. Room clearing is a matter of minimizing chances, not eliminating them.
I call it the hedgehog. It lifes here now. Very good at trench clearing too. https://imgur.com/a/hedgehog-De90woA
Bonuspoints if it can be transported and drone dropped!
Sounds like a stairwalking robot that has 360° cameras would be pretty useful
Maybe you implied it, but adding to the 'going up' case, there is bullet ricochet -- also reffered to as skipping or bouncing bullets -- that makes it so that bullets will ricochet out at a shallow angle and ride out the wall or floor, making your head hittable without even aiming at it. Here is a instructional FBI video talking about it.[0] And a more modern take.[1]
[0] https://youtu.be/7tUW0cUkNv0?feature=shared&t=240 (I think the date is wrong, other places mark it as being from 1974-5)