A text to my wife: Gemini is in the middle of the track, so I opened the gate for him.
Back from her: Hmmm, OK
She was confused, and should have been since she was away at a horse clinic for a couple days, and she had Gemini with her. He's the black horse we have, and the only black horse. Or so I thought.
My wife had agreed to be a middlewoman between someone selling a horse, and someone looking to get a horse for a friend. When the lady number two saw the horse, she felt it was "too much horse" for her friend. So Starlet, the horse for sale, stayed at our place. I had assumed that Starlet would be heading back to the original owner any day.
Apparently I forgot what an assumption really means.
Starlet was in our pasture for a few weeks when I mistook her for Gemini. I know one's a boy and one's a girl, but I didn't check that closely. I was heading out one day, saw a black horse in the middle of the pasture with no water, and opened the gate.
I wasn't thrilled with us going from three to four horses, but my wife pointed out that we'd been dealing with it, and the chores, for a few weeks and I hadn't noticed. At that point I didn't have much of an argument left.
It pays to be able to tell your horses apart.