I'm a hardwood floor guy, and I can say this is one of the better homeowner jobs I've seen. Next time, you shouldn't end up higher than 100 grit on a floor and you really shouldn't sand the finish or stain. Its not a tabletop and the finish, if applied correctly, won't leave noticeable roughness. I'm not sure of the exact types of finishes you used as I've never used them, but it is possible that it may start to peel up after a few years due to the high grit sanding. DON'T try and refinish that spot, or peel back the finish, or anything like that. You'll never match the color of an aged stain and finish, also a new finish coat will stick to the stuff peeling up. Get a hardwood guy to come out and get an estimate for your house. All the good contractors will do this for free. He will either say he can fix the spot or you'll need to refinish. Its worth the money to let him fix the spot if he can. There aren't enough trade contractors out there, and there is a 95% chance he isn't trying to screw you. I have more work than I can handle so I would much rather do a 2 hour fix than to tell another client, "I can't fit you in I'm book. How do you look in 3 months?" Every other floor guy I know has the same issue. IF it even is an issue.

To others looking at doing this:

This is an overly simplified explanation of my professional process:

-Overview- Sand, clean, fill, Sand, clean, sealer coat, clean, finish coat, clean, finish coat.

Drum sander - 45deg crosscut @ 60 / straight cut at 80

Edger - 60 grit around the edges, 80 grit ONLY if its a very soft wood, the old finish is gummy, the previous sanding job was crap, or if my previous 60 grit cut was crap

Now use wood filler to fill the floor, let completely dry, and fill in larger cracks. Edge each room @ 100 grit. THEN straight cut with the drum sander AFTER edging. Scrape the corners of old finish and hand sand with 100 grit. Then buff the whole floor with 100 grit screen. Vacuum. Vacuum again. Apply stain, sealer or oil. Let dry overnight. Vacuum. First coat of finish. 250 buff screen entire floor VERY lightly and quickly. Vacuum. Apply 2nd coat of finish. This contradicts what I said above, but I rarely see professionals do this correctly, much less the first and only time a homeowner finishes their floor. You're just scratching it so the next layer adheres better and doesn't peel up as time goes on. If applied correctly this will likely won't happen without the 250 grit buff. Just read the instructions. If its humid, or cold, you want to give yourself an extra half day to let each coat dry. The problems with floors arise when you rush the last few steps or do it poorly.

Notes: Vacuum between every step. Small pebbles and bits of wood, finish, nails, pennies, thumbtacks, hotwheels(its happened) will find their way under your sander from places you least expect. Not only will it ruin the rubber sanding pad which will create an uneven sand on the whole job, it will scar the floor. Pennies are particularly scary because they get caught under trim then turn into bullets very fast.

Don't sand to a higher grit. You burnish the wood and it doesn't allow the sealer coat to soak into the wood creating a deep enough layer which may turn into adhesion problems after several years.

NEVER sand to bare wood after the first finish coat. If you need to fix something in the sealer coat, do it then. Scrape out the imperfection, hand sand a small around around it to bare wood and reapply the sealer. Let it dry COMPLETELY. This is VERY important for the finish. Do this sparingly, and always use LIGHT pressure. You can sand more later, but if you sand off too much the color won't match.

If stuff is stuck in the finish, CAREFULLY pick it out, and don't let the finish peel up. Get a 120 grit+ and do 5 seconds or so of LIGHT hand sanding on the spot made by the pebble / fly / wood.