I’d file this one under “common errors of fantasy”, except that it’s more ubiquitous than that.  Call it a common failure of visualization, maybe.

The thing is, no matter where your scene is set, the light is going to be coming from somewhere . . . and a lot of beginning writers forget that you need to keep track of such things.  If the action is taking place outdoors in broad daylight, or indoors during the modern era, illumination can be more or less assumed.  The reader will need to know if something happens to make the light diminish or go away — power failure, dense cloud cover, a darkness at noon of biblical proportions — but unless there’s something special about the source or quality of the light, it doesn’t need to be mentioned.

Outside of those circumstances, though, things change.  If the characters are operating at night, or underground, or in deep woods, or indoors during any pre-electric era, then you-the-writer are going to have to be aware of how much or how little light there is available for every scene.  A cloudy night, a clear night in the dark of the moon, and a clear night with a full moon are going to have different light levels — enough to make the difference, for instance, between your characters being able to operate unseen (if they can avoid tripping over tree roots and large rocks) and being plainly visible to any alert sentry on the castle wall.

Meanwhile, inside the castle, the wizard perusing his books of lore by the flame of a single candle isn’t going to have all that much light to work with either.  As a writing experiment, try lighting one candle in a dark room and then describing how much you can and can’t see.  If you’ve got access to an oil lamp, try that sometime.  Life without incandescence or fluorescence looks a lot different.  (And it’s no wonder that the reading-light spell is a classic beginning wizard’s trick in so many fantasies, right after the fire-lighting one.)

And if your characters are going exploring someplace where they’ll need to take their light source with them, don’t forget that someone in the group is going to have to be carrying the torch or candle or lantern, and that person won’t have both hands free to do other things.