Sometimes when you are standing on a stair, if you try to place another one right above it, you will succed and half of your body will be inside the stair.
This happens mostly when I am standing on the second step and try to place the stair a litle to the side.
Duplicate of MC-2208, please use the search function to see if your bug has already been submitted. Currently over 58% of tickets are being closed as duplicate.