Continuity mistake: The first time Juno is seen at school while visibly pregnant, she is talking to Bleeker who is holding a box of doughnuts. In shots from behind Bleeker the side of the box is see-through, while in shots from behind Juno the side is white, and it continues to change back and forth throughout the entire scene.
Continuity mistake: When Juno is walking to Women Now and runs into Su-Chin, as they talk, Su-Chin's sign is constantly changing position. When they show Su-Chin the first time, her sign is high near her head, then when they show her shoulder you can barely see the top of the sign. This continues for the whole scene.
Continuity mistake: In one scene Juno puts a large number of Tic-Tacs in the letterbox outside Paulie's house. This appears to be at the street end of the path from the front door to the "gateway" in the front fence. In the last scene, Juno and Paulie are sitting on the front fence on either side of the "gateway". There is no letterbox visible even when the camera pulls back to reveal almost the entire frontage of the property.