What I expected to happen was...:
I expected to be able to access chests and crafting tables through the gap in an upsidedown stair.

What actually happened was...:
The hit box for the stair prevents clicking through the gap. if you rotate the stair 90 degrees around the z axis you can click through the solid part of the upside down stair block.

Steps to Reproduce:
1. Place an upside down stair over a chest or crafting table with the gap facing you.
2. Try to access the chest.
3. ...profit