Due to how half blocks work, blocks suck as stairs, and how they allow you to move down half blocks without needing to jump, this creates a problem that is often present when placing stairs as roofs.

As you move across the top part of the stair (as you can't place blocks from the lower part of the stair) you will sometimes find yourself descending down the stair, and because of your movement, you don't land on the lower half of the stair, but instead you fall off, and possibly to your death.

This problem is often present with builders, and I would consider it a bug, even though it is working as intended. Sneaking should be a safe way to place blocks while removing the danger of falling off, and this is one of the biggest things builders need "shift" for, and this is defeating the purpose of sneaking.