As I was joyfully playing in my minecraft world, just decorating my Christmas house and all, i came through this (see attached picture nr 1). As you can see, my crosshair is pointing at the prismarine stairs, but the selected block is the one behind it. If I try to break it, it will not affect the block my crosshair is on, instead, it will break the stair behind it. This occurence only happened once to me and it seems to only work from this certain angle and probably the same type of stairs. Its not a big issue for me, but its been confusing for a few seconds until I realised its a bug.

My furter observations showed that it affects all kinds of stairs, and stairs only. The stairs have to be placed exactly like in the picture for the bug to appear. You have to place one stair normally, and the other one next to it facing a different direction. 

In screenshot 2, the stair seems to lose its hitbox and is undetected by the crosshair. Instead the block behind it is selected. I cant unsee it after I found it out.

If this needs real video footage, i can make one.