When talking to Richard Grieco after he runs into their van, the fake plant foliage the brothers are covered in disappears and reappears between shots.
When Steve is cheering up Doug with Cambi's number, he picks up the frame with his right hand and in the next shot, it's in his left, then right again.
During the Staying Alive number, Doug's headphones go from on his head to round his neck, back to on his head again.
On their way to their father's "fake plant shop", the Butabi brothers pass the lamp shop on their left. This shop is supposedly right next to their father's store (to the left), yet they keep walking for a few more minutes. Then we see them walk into their father's store coming from the right, when they should've walked in from the other direction.
After Steve and Emily get engaged, Emily starts putting several of Steve's possessions in a cardboard box to throw away (his going-out clothing, a disco ball, etc.). Emily also grabs his silver chain necklace, tears it off, and throws it in the cardboard box with the rest. Steve then has his necklace on shortly thereafter.
When it says Freeway 6:02 PM, it is obviously clear that two body doubles are driving in the van instead of Will Ferrell, and Chris Kattan.
When Steve and Doug are in the van, the hanging flowers behind their heads change color from scene to scene.
When the woman on the balcony is waving towards the brothers, she says "not you", but her mouth never moves.