The correct answer or at least the canon answer @bowlturner has already mentioned. Doing the whole room is the best way to do things. That said...its more expensive and more time-consuming.
If your problem is in the middle of a floor, in a high traffic area, spread out over a decent sized area (2 sq feet ore more really) or otherwise blatantly obvious...yeah you are going to have to sand everything (it would be best if you could post a picture of the area you need to fix).
Now...all that said I have touched up spots on floors before...and while they personally drive me insane because I see every minute flaw, most don't notice.
For a relatively small flaw/area here is what I have done (and to all the perfectionists, no...I am not proud of this :) )
Tape around the area as tightly as you can, meaning don't make it a square. Give about 1/2 inch between the tape and the flaw(s) all the way around. Make sure you use quality tape that will not pull up.
Sand...yeah I said it...sand. Now. How much you sand depends on the depth of the flaw. If it is just a flaw in the finish...ignore everything I said, get some floor wax and buff the crap out of it. Problem solved.
If it is deeper, meaning into the wood, sand as needed to smooth things out, apply a stain absorbing filler if necessary. Sand the entire taped area at least enough to roughen up the finish.
Clean really really really well.
Apply finish. I actually use an aerosol spray finish most of the time, they usually come in satin, semi-gloss and gloss. Spray the finish and let it dry completely, use thin coats of finish.
Pull the tape painter style (back on itself rather than up).
Sand with a smooth grit paper or sometimes you can get away with a 0000 steel wool...be gentle either way.
Once that is done buff/polish the floor and it should blend in pretty decently.
This method is all about touch, go light, if you're heavy handed it'll just be a pain