The most stupid thing I've done / do as a pilot? It would have to be assuming the intelligence of others.
For Example: Was told my pax were waiting in the customer lounge. Introduced myself and said "Hi, you must be Shane" to the passenger. Loaded up him and his party. Just about to lift when I notice someone from the ops team waving wildly at me. Turns out they're not my pax.
Said to the guy "I thought your name was Shane", he says "No, I'm Kieth"!
Mistakes: 1. Believing the first person who told me they were my pax. 2. Believing the customer would question me if I addressed him repeatedly with the wrong name.