I have refinished the wood floors in my house with the exception of 2 bedrooms, which share a hallway with wood floors. I am redoing one of the bedrooms, and the company that is doing the floor assured me he could match the stain so it would match my floor in the hallway. I just got home and the stain he used is darker, and he got some outside the bedroom into the hallway floor. It looks sloppy. Had he not got some into the hallway, and just stayed within the bedroom it would be okay. It is drying now, and he is coming back tomorrow to finish up. Should I be worried? Will the stain lighten overnight? Is there a way he can fix it?
Matching stain is very difficult. Seriously, I heard of one guy who markets himself as the "Wood Whisperer". When guys start calling themselves a wood whisperer, you know you are getting into the black arts.
Typically the average floor installer will use a wood wheel from his manufacturer and pray.
That said, you will not know how good the match is until it ages a little bit and fades, so you have to give it some time. Basically, what this means is that in 3 months if it still doesn't match you can call the guy up and yell insults at him.
You cannot "fix" a stain. You can try re-finishing, but you are rolling the dice even more on that one, because then the wood wheel will not work anymore. The wood wheel is only for fresh installs. Refinishing requires a "wood whisperer". Also, refinishing is expensive because it involves sanding.
Note that finishes can vary under different lighting conditions, so that a finish that matches in daylight, could look totally different at night under incandescent or fluorescent light.