steps to reproduce:
1. build the building in the images (the block near the slab is not needed. it's here to show that it must be an upper slab)
2. get on a horse
3. ride and get on the stairs
4. you are now crawling on the horse
NOTE: removing both walls will make you sneak instead of crawling

a little adjustment is sometimes necessary

after doing that the log says that the horse moved wrongly