Doors at the top of stairs
This may seem like a no brainer. However, it is a common mistake. Doors at the top of stairs (without a landing) can not swing into or over the stairway. The reason- If someone is walking up the stairs and happens to be at the top; yet another person on the opposing side of the door has has decided to open the door to go down. The result could be a tumble back to the bottom of the stairs.