When punching and plugging holes in a wall, the "column" that forms on either side of the hole sometimes
1. does not extend all the way to the ground
2. starts at ground level and stops in the middle of the wall.
I have observed 2. only once, in this video, and have not been able to reproduce it so far.
Reproduction for 1.: (This is the only reliable way I have found so far, more may exist.)
- build a wall that is at least 5x5
- punch a hole in the wall, then one to the left or right of it, then a third one below that
- place a wall in the hole you punched second
- place a wall in the hole you punched third
- the colum on that side of the remaining hole should only be two blocks long and then stop
This is infinitely scaleable along the Y-axis, just fill in the missing walls top to bottom. The first three screenshots show these reproduction steps.