I encountered this bug when riding boats and horses, but I do not know whether this applies to other ridable entities.

Occasionally, when dismounting ridable entities, in my case boats and horses, the entity I was riding will disappear. Boats and horses are no supposed to do that. This has happened to me three times within a few hours, twice with a boat and once with a horse. When this happened to the boats, I was dismounting while moving. The horse disappeared when I ran into some water and was thrown off. 

The boats going missing is annoying when it strands me in the middle of the ocean. When the horse disappeared it took the saddle with it.