I Look Really, Really Good on You
"Derek already knew he was going to regret this decision. He never should have listened to Lydia. It was a stupid costume idea, nobody was going to get it. But she was insistent. And there was something in the tone of her voice that made Derek oddly uncomfortable.
'I’m not wearing those,' he’d flat out refused the pile of immaculately folded clothes she proffered to him. She narrowed her eyes. Something in Derek’s chest went cold as ice as a result.
'You will, and you’ll like it. Or so help me, I will find a way to poison your protein bars with wolfsbane'."
a.k.a. the one where Derek doesn't think Lydia's idea that he dress as Stiles for Halloween is going to have the effect she seems to think it will. Until he sees Stiles' costume. Then it happens to him instead.
Telsu