That happened to me once as well, a I can't remember if it was my husband or my son that locked up the coop for the night, but they missed her. She was shut out all night and she was sleeping next to the coop in the morning.
The only reason I know it wasn't me is because I do a head (or tail count depending on which way they are facing me from the roost) when I lock up and they won't do that. They get confused when the hens start moving around and they think they all start to look the same.
For some reason my black copper maran cockerals use to try to hide around the corner from the coop and not go in at night when I wanted to lock up. They were very tough to catch too. Its the first time I ever had roosters do that. The roos are usually the ones that keep circling and nudging the girls to lead them into the coops at night. That is the one thing I like about roos, the nice ones anyway, they know how to treat their ladies and really watch out for them.
Except for this one lout I had, if a hawk started circling he would be the first one cowering in the hen house while they other rooster would be rounding up the girls and protecting them.
Anyway, this had been going on for months until about 2 or 3 weeks ago and now they finally go in.
Its weird because I have never had issues with any of them going in at night until I started using the floodlights.
They all seem past it now though so I just have to go out and lock the doors again.