For Automatic1111, the easiest fuckups are messing with the scale and not using a model that can handle inpainting. Then there are the unintuitive "fill" radio buttons that I don't really understand myself (what they do is obvious; why you'd use them is not).

InvokeAI has a much friendlier UI, inpainting is easier, and the platform is more stable, but is lightyears behind in plugins and functionality.