I was on the train with two friends from university. I hadn't been on a train since I was very young and I thought you had to show your ticket to someone on the way off the train (based on a very vague childhood memory from the last time I was on a train), so I showed my ticket to some random person on the way out. My two friends both noticed and laughed at me for it. I just wanted to die there and then after I realised.
Of course, now I know how the ticket system actually works, so something good came of it. Call it a "learning experience".