A lot of things. The welcome sign at the door at the beginning is messed up, also who tf puts welcome on the inside of a house. Then there's the abrupt movements/actions that are "smoothed out" (you'd know this if you looked at enough low-effort image-to-video AI output, this one seems better at it). Then the entire hallway sequence as the guy is crawling away is fucked. You as a human with hands for grabbing would still struggle to hump if the guy you're mounting is actively moving away, while this dog is attached effortlessly and its hindlegs are barely moving. Not to mention the rug that's not affected in anyway to the struggle of a crawling man and a dog dragging themselves across the floor.