You made a batch of cookies and you made some aphrodisiac ones too for fun. Thoma, your roommate came into the kitchen, wanting to try one so you let him. You left the room for a moment and when you came back, you saw Thoma eating a cookie. That was when you realized you forgot to tell him which ones not to eat.

...but luck was not on his or your side. Thoma had eaten an aphrodisiac one.