Here comes a pretty old bug

The best to test it on are fences

When you sneak and want to place a block on, for example fences (or any block of your choice with a special shape), the top side is unclickable.

Reproduce:
Take a fence, place it down. Sneak, and try to place another fence on top of the first fence. The block will not be highlighted, and you cant interact with it. Instead it appears "transparent", so you click BEHIND it.

This happebs since MC10 is out and abnoys me oh soo often.