When Angelica is getting in bed, her pillow is vanished, but then it appears.
When Angelica is punching her pillow, her shoes are off. But when she's out of bed, her shoes are back on.
In Slumber Party when Tommy wakes up and yawns after feeling better he is drawn with a full set of teeth, at this point in the series Tommy has no teeth.
The way the positive copy of "The Land Without Smiles" melts is unrealistic.
In reality, the sound should have been warped (without the strange garbled sound) with the filmstrip slowing down for the next few frames before it melts. When it melts, the film base's melt should have been in a darker color instead of white, and the telecine should have been on with a white screen. The frame should have also started to melt with one sticky hole before it keeps spreading the melt of the frame.
At the beginning of Slumber Party, there's a clock that has no hands.