James Doohan ("Scotty") lost his right middle finger during WWII. Most of his scenes are shot to hide it. In scenes with a close-up view of Scotty's hands (operating the transporter for example) someone else's hands were always used. However, it is noticeable on a few rare occasions. The most obvious example is "The Trouble With Tribbles" when Scotty comes into the Rec Room with his arms filled with Tribbles. Take a close look at his hands. You can clearly see he's missing a finger.