Someone already mentioned this, but when the camera is too close to the actor's dick. It makes me feel like my eyeballs are sticking out of my tummy, which is sort of neat but just not the sort of thing I generally jack off to.
In cowboy, when the girl's head is high enough that the top of her head is outside the camera's frame.
When the guy uses his hands, and they're super unlike my own (for instance, very hairy). But since everyone's hands are different there's really nothing that can be done about that.
When I'm watching a JAV video and I suddenly notice everyone there is just unnaturally small, including "my" own hands. Not just small like "many Japanese people are smaller than Americans," but small like "they've all been shrunk and human skulls really don't get that small. Once seen, hard to unsee.