Jaws
Technically, extras aren’t supposed to have any lines. Every so often, though, one gets promoted to a speaking part. It doesn’t always go spectacularly well, but it never went as spectacularly horribly as it did in Jaws.
So, a shark has been fished out, and some townsmen come over to appreciate it. One man, known as Pratt in the credits, keeps wondering what kind of shark it is.
When Richard Dreyfuss’s character replies that it’s a tiger shark, ol’ Pratt replies, in a weird cartoon character voice, “A whaaaaat?” There’s a reason the actor, Dick Young, has literally never appeared in anything else.