After watching the Real Hustle on Holiday, I learnt not to give my passport to a policeman, not to pay for a ticket when parking, and never give my room key to a member of staff at the hotel...
Never give passport to a policeman because they might not be a policeman and will steal your passport.
Never pay for a ticket when parking... Well, if it mentions something about paying by text, then ignore it.
Never give a member of staff your room key because they can clone your room key and rob your room when you leave.
I don't need to worry about the first 2, but the last one I should when I go to Spain. I think I may end up hanging a "do not disturb" sign up whenever I leave the room now.