It's bad when you realize you have it in you--when the muse is sitting nearby and not fidgety or flighty or simply unavailable--to knock out a near-finished short story in four hours.
Not bad when you can do it. Bad when you realize you can do it.
Because then you feel like you can procrastinate. And your creative muse is off getting her nails done somewhere. Or building sandcastles. Or making omelets with spinach and tomatoes and fresh thyme.