In the backyard barbecue scene, Paul's hot dog shows just having ketchup on it, to having both ketchup and mustard, to just ketchup again.
In the scene where the central characters have dinner at Paul's house, Paul pours wine into his glass. As the scene progresses Paul's glass changes from empty to full about 6 or 7 times.
When Laser and Joni hug at the University, Laser hugs with one arm up and one arm down, and so does Joni. But in the next scene, Joni's arms are both up, and Laser's are down.
In the scene concerning Tanya and Paul, as he discusses the phone call from the sperm bank, the necklace Tanya is wearing disappears then reappears.
During dinner in the beginning of the movie when Jules talks about the truck, she has a cucumber slice on her fork. In the next shot, it is gone.
When Jules and Paul are in Paul's garden for the first time, crew members and equipment are reflected in their sunglasses.
(at around 24 mins) While Mark Ruffalo's character is on the phone walking through his garden, a Star Waggon is very clearly seen centered in the frame behind the foliage.
Laser tells Paul that Joni got the National Merit Scholarship for science; however, National Merit Scholarships are not awarded in any specific categories. Candidates are chosen because of high scores on the PSAT, which does not include a science section.