We had cat urine on our floors which did not come out with the floor refinishing we had done in the old house. They ended up being noticably darker areas when finished with a clear coat.
We have a couple of dark water marks on the floors in this house which the pro that I called (he has an excellent rep) told me wouldn't come out. He suggested having the floors stained dark to camoflage the marks. Now, I thought a wood bleach after first sanding would take out the dark stains, but the refinisher pro didn't sound hopeful. He said I might end up with areas much lighter than the rest of the floor.