I've had this happen a couple times, mainly on the ocean. It's very confusing trying to move ships that aren't where they appear to be. Also, a landship belonging to an AI opponent remained next to my settlement after we declared peace. Except it wasn't really there. When I moved a unit onto the tile, the landship would disappear, but if I moved off, it reappeared again. I learned to ignore it. Not something that should be happening, though!