(Updated to use suggested format)

Affected version: 1.5.2

Summary: When attempting to ride a tamed, saddled horse if you are holding a fence when you left-trigger, it will place a fencepost in the ground beyond the horse (and you will ride the horse as well).

Steps to reproduce:
1) Beg, borrow, or steal a tamed, saddled horse
2) Have a fence in primary hand
3) Attempt to ride horse (left trigger on Xbox)

Expected result: you climb on the horse

Observed result: you climb on the horse and also place a fencepost in the ground just beyond the horse

Pictures/video attached: No

Note: I tried a couple of other blocks and they did not show similar behavior, i.e. it did not place a dirt block if I was holding dirt.

That is a dirty lie. This does affect other blocks; I must not have been pointing at a close-enough spot of ground before. It definitely will drop a block of, say, cobblestone if you try to ride the horse with cobble in your hand.