Summary: Psychic paper is supposed to get the Doctor out of difficulties, not into them.
Sometimes psychic paper caused more problems than it solved. Needing to get aboard a spaceship to search for a dangerous shapeshifter, the Doctor flashed the slip of paper at the ship’s captain.
It was supposed to show the man what the Doctor wanted him to see, that he was a customs inspector, but the captain must have had other things on his mind.
“You’re a qualified mechanic? Well why didn’t you say so? The port engine’s been running rough for weeks and my engineer has no idea what’s wrong.”
The Doctor sighed. “I suppose I’d better take a look then.”