When Clara is chasing Snoopy and Woodstock, she is holding the leash in her hand. When she is standing at the top of the stairs, the leash is gone. In the next shot, when she is sliding down the banister, she's holding the leash again.
When the young girl is writing her letter, she writes two lines, then sucks on her pencil. When she goes back to writing, she writes the second line again.
When Snoopy gets Woodstock to type the letters to Charlie Brown and his friends at the end, the dimensions of the items on top of the doghouse change from shot to shot. The most noticeable of the items is the typewriter which takes up different amounts of real estate on the doghouse from shot to shot.
Snoopy puts his food dish on his head and he is seen without it in the next shot, but it doesn't show him take it off.
When Clara is singing and dresses Snoopy up in a dress, she pours the tea and over fills the tea cup. It spills over onto the table and drip onto Snoopy's dress. She gets upset about his dress being "dirty" and spanks him. When Snoopy gets away from Clara and runs to the phone, his dress is clean.
On the Monopoly board, Free Parking is where In Jail/Just Visiting is supposed to be.
When Peppermint Patty and Charlie Brown ride on the Ferris wheel, the ride is running the wrong way.
It should run clockwise, not counter clockwise.
When Clara starts choking Snoopy as she catches him his nose disappears for a few frames.
The gang is playing Monopoly and Charlie Brown rolls a seven. The shot cuts to him when he says, "Seven!" and you can see that his mouth is not moving.
Lila should've already known about the "No Dogs Allowed" sign at her apartment, and also for the fact that that's why she had to give up Snoopy in the first place.
While playing Monopoly, Lucy rolls a 12, but she doesn't move her pawn the full 12 spaces.