When working in villages to increase village size, I have found that some mobs end up on top of buildings or in structures that their abilities would not allow(iron golems in areas of a house where the stairs are too small for them to fit, villagers appearing past village walls when all doors outside the wall are iron and button operated). They will often stay there until moved, or appear in different areas. This may be in part due to the large village size I have (at least 27 residents) but it could also be a new bug.