I remember reading at least the first chapter of a manga where a girl gets transported to another world in a European setting fantasy world. But she transports to a man's room and he mistakes her for a courtesan someone put there, and she goes a long with it cuz he's handsome and they spen the night. They wake up and he discovers she is a transmigrator and apologizes. That's all I remember but I know it was a manga in black and white.

Why do I feel like I have encounter this type of scenario before? ".... he mistakes her for a courtesan someone put there, and she goes a long with it cuz he's handsome and they spen the night. They wake up and he discovers she is a transmigrator and apologizes."

Could it be +a no Tachiichi?

Rokkon Shojo
TeppenMezashiteTondeAruYo
Let justice be done, though the heavens fall

That's what I remember too!! I don't think it's +a no Tachiichi unfortunately. But yeah he mistakes her and then apologizes the next day since she transmigrated into his room or something by accident.

I found it! It only has 2 chapters translated a far as I can tell, but this was it! It was mainly just bothering me that I remembered it but couldn't find it.

Isekai Trip Shita Sono Bade Taberare Chaimashita
https://www.mangaupdates.com/series/xrd5dks/isekai-trip-shita-sono-bade-taberare-chaimashita