Yup, one time isn't a problem, even if it was all night. But even small light leaks on a consistent basis can ruin a crop.

I've read that the light level needed for the plant to "recognize" the light as daylight is about 3x as bright as a full moon. But I also wonder if plants that have been grown in complete darkness during the night cycle are more sensitive to light cycle interruptions, and of course some strains might be more or less sensitive...