Another time I DM’d I had an adventure that started in town with a festival/carnival, and one of the major NPCs with the festival was actually the bad guy. The clue was the way he laughed. However, it was not until the party was in the dungeon that I realized that when I go to the note about using this laugh in the dungeon, I had not given them the clue in town.
In my inexperience, I gave it away by describing the laugh in exacting detail, as one they heard at the carnival, so it was a give away that ruined the fun of the players figuring it out.
There are just certain things you cannot forget in an adventure when you are a DM, especially when it is one that you designed yourself.