Just that it's quite hard to stain specific spots without leaving tide lines around the area. Better to re-stain the whole surface, but start and finish with the problem areas with lots of stain on the rag (or sponge etc) and spread out from there with the now drier. but still wet enough to stain, rag. You may have to do it a couple of times to match the stain intensity. The already stained areas shouldn't get any darker.