Doesn’t happen with real models

That actually happens quite a lot. Hollywood movie poster are especially full of it, as they rarely do specific photoshots for the poster and instead just copy&paste together whatever random images they can find. So you end up with stuff like the 300 poster where the sword doesn’t attach to the handle. Magazine covers adding an extra hand or leg isn’t all that uncommon either.

And that’s the expensive stuff, once you go into low-budget productions like self-published book covers, it’s all just crude stock image copy&paste at best.