When Juno goes to visit Mark for the first time, she is wearing a skirt over her jeans, when she returns home and has the conversation with Brenda about marital boundaries, the skirt is gone. Immediately after, the movie shows Juno going to visit Bleaker and shows her taking off her skirt in the car before she goes up to his front door.
When Juno encounters Vanessa outside in an elevator the mall scene, Juno states Vanessa's name in surprise but she mouths Jennifer (the actress' name rather than the character's name). The 'J' can still be heard.
Juno fills Paulie's mailbox with his favorite orange Tic-Tacs, telling him that "you can never have enough of your favorite one-calorie breath mint." Tic-Tacs have two calories each (and were famously advertised as such).
When Juno enters labor, she shouts "Thundercats are go!", an error or more likely a mixed reference to both the American cartoon series "Thundercats ho!" and the British Supermarionation series "Thunderbirds are go!".
Juno says "...and always smells like methylmethacrylate..." (according to the script.) She pronounces it "methyl methacralyte". Methyl methacralyte isn't a real chemical compound. Methyl methacrylate is used to make plastics, like plexiglas or PVC. Methyl acrylate is a chemical compound used to make nail lacquer.
When Juno goes to the track to tell Paulie that she is in love with him he asks if she was the one who left the orange Tic-Tacs in his mailbox and why she did it. She replies that it is because they are his favorite and you can never have enough of your favorite one calorie breath mints. Tic-Tacs actually have one and half calories or more, with Orange Tic-Tacs having about 2.5 calories.
When Juno turns out of the way to cry, there's a freight train moving along the highway at the beginning of the scene. And in the end of it, when Juno drives off, the box-freight cars keep showing up. But in-between, during the shots inside Juno's car, the train is gone, and you can only see the field through the driver's window.
When Juno and her Dad first visit Mark and his wife, the view from outside the front door shows the Dad wearing a brown baseball cap. When the door is opened the view from inside the house shows the Dad with the cap in his hand. When they leave together a little while later the Dad is holding a black baseball cap, not brown.
When Juno enters the drug store for a pregnancy test a guy and girl are shown outside the window. The guy was sitting on a statue and talking to the girl. When Juno goes inside the store, the window is shown again from the inside but the girl, guy and statue have disappeared.
When Juno searches for a piece of paper to write on, she finds a crumpled up Jiffy Lube receipt. When Vanessa grabs the note from Mark, the receipt looks like it has never been crumpled. When we later see it framed, it looks crumpled again.
When Juno is at the clinic, the jar on the desk holding condoms first appears that all the condoms are purple. Then when the shot changes to behind the desk, some yellow and blue condoms can also be seen in the jar. Then the shot changes back to next to Juno and the jar is back to all purple condoms.
The bottle of Sunny D Juno is drinking in the opening is the type of bottle found in Canada with a rectangular shape rather than the circular shaped bottles found in the US, and the size is marked on the bottle in liters instead of ounces like the US bottles.
When Bleeker is talking to Juno in the hallway at school while
holding a box of donut holes, the box changes positions in his hands every time the angle changes. In his DVD commentary, director Jason Reitman explains that this was an intentional cheat (one that irked his script supervisor!) to keep the donut holes visible through the box in both camera angles.
When Juno goes to visit Mark and Vanessa to give them the ultrasound photo, Mark tells Juno that Vanessa is working late to accumulate time-off for when the baby comes. As there is snow on the ground, it is clearly winter. However, when Vanessa comes home, it is still light outside. Minnesota in the winter is dark around 4pm.
The small quartz decorator clock in the kitchen of Juno's house, visible over Mac's right shoulder in several scenes, has a large, red sweep-second hand that reveals that the clock is obviously not running, presumably to preserve continuity and avoid having to reset the hands for every retake.
When Juno is visiting with Mark and Vanessa comes home, she seems surprised to see Juno there, but when Juno and her dad visited, they parked the same blue minivan in the Loring's driveway, so Vanessa should've known either Juno or someone in her family was there.
The goof item below may give away important plot points.
Mark suggests that he and Vanessa use the same lawyer for a collaborative divorce. While they got it right that collaborative divorce is non-adversarial, each participant must have their own lawyer in collaborative law.