When you're renovating, save and recycle old hardwood floorboards for patching and repairing. Using a pry bar at each nail, carefully remove the board to minimize splits and damage to the wood. Tap out the old nails, then reinstall the flooring as needed, staggering all joints. When it's refinished, you won't be able to tell where the repair was made.
For more on flooring, consider:
Wood Flooring 101
Quick Tip: Reclaimed Wood Floors
Installing a Reclaimed Wood Floor (VIDEO)