While standing next to a solid structure above head height, looking down, one can jump and place a block underneath your feet. When doing so, an additional block is placed atop the surface of the adjacent structure.

Initially I thought this was duplicating the block, but closer testing revealed it is taking the block from inventory.

The screenshots below demonstrate. I am standing in an empty space in shot 1. Shot 2 I create a stack of 3 blocks. Shot 3, I stand adjacent to the stack of 3, looking down. Shot 4, I have jumped and placed a block, while looking down. Shot 5, I stand back from the pillar and see an additional block was placed atop that pillar, even though I only attempted to place one underneath me.

I tested this in creative mode and was able to reproduce it with an adjacent stack 4 and 5 blocks high, though it seemed harder to reliably reproduce. It seems to require a specific cursor placement at the base of the adjacent blocks.

Here is a video of this behavior: https://youtu.be/sQM20ZcgWMY