All the furniture in Woodstock's new birdhouse disappears when Snoopy breaks the birdhouse after his nose gets stuck.
When Snoopy and Sally are trying on hats, a brown hat that Snoopy takes off turns purple when it hits the ground.
As Lucy is putting out the eggs for the Easter egg hunt, all of the eggs in her basket are of solid colors but each one she takes out is elaborately decorated.
When they go to the shopping mall and see all the Christmas decorations, Lucy tells Linus, "I told you this was the gift-getting season!" She did say that earlier, but when she was talking to Schroeder, not Linus.
When the gang visits the mall, a sign proclaims it is 246 days until Christmas. Assuming the film's events take place in the year of its release, this means the date is April 23rd, 1974. Easter fell on April 14th that year, so Sally is searching for Easter shoes 9 days after the holiday.
Incorrectly regarded as goof:
While Snoopy and the gang are at the mall, the gang takes one escalator up and Snoopy takes the other down. As they do, they cross paths, even though they're going in different directions.
This is a visual gag intended to be silly.
Snoopy's mouth does not move when he shouts, "Hey!" to the dancing bunnies.