According to Sean Astin, he was allowed to keep the treasure map used in the film. Several years later his mother Patty Duke discovered it, thought it was just a crinkled piece of paper, and threw it in the bin.
When rocks are falling from the cave ceiling, Data screams "Holy S-H-I-T!" Ke Huy Quan said he spelled the expletive because his mother made him promise not to use any bad language in the film.
(at around 46 mins) When the Fratelli brothers argued, Anne Ramsey really slapped Robert Davi. She was told to hit him as hard as she could.
The pirate ship was entirely real. All the shots were filmed in the ship. After the film, it was offered to anyone who would take it. No one wanted it, so the ship was scrapped.
(at around 34 mins) Robert Davi, who is a trained opera singer, came up with the idea where he sings to Sloth the aria "Una furtiva lagrima" ("A single secret tear") from Gaetano Donizetti's opera "L'elisir d'amore" ("The Elixir of Love").
Jennie Lew Tugend: (at around 1h 40 mins) assistant to director Richard Donner, has a small role as Data's mom.