Be My Valentine, Charlie Brown (1975)

Continuity mistake: As the animated film opens, Charlie Brown's mailbox has a small red flag that is attached to the rear of the mailbox with the flag facing up. At the end of the film, the flag is attached near the front of the box and it is facing down.

Lucy van Pelt: What are you doing, Charlie Brown?
Charlie Brown: I'm waiting for valentines.
Lucy van Pelt: Oh, well. Good luck.
Charlie Brown: Thank you.
Lucy van Pelt: You'll need it.
Charlie Brown: You didn't have to say that!