When the character Andie Anderson says, "I mean this is Defcon 5, and I have to do something truly appalling," she really means she is at Defcon 1 or 2. Defcon 5 is the lowest level of defense readiness. Defcon 1 is the highest.
Near the end of the film when Ben is on his motorcycle watching Andie put her luggage in the taxi, there is a quick shot of a traffic light where it is raining, but in the rest of the scene it is sunny.
Just before Andie surprises Ben at work with Krull, Tony hands Ben a mug and he begins to sip from it. Ben has a black mug and Tony has a white one. In a subsequent shot they've switched mugs - Ben is now holding a white one and Tony has the black one.
After Ben tells Andie he wants to go to couples therapy, she walks away down the street and waves back at him. When she's walking toward the camera, she waves with her right arm while holding the plant with her left. When the shot changes and she's walking away from the camera, she's waving with her left arm.
When Andie arrives at the DeLauer party, she is wearing no earrings. Later, after Andie and Ben leave the ballroom, she is wearing the necklace and earrings. The security only asks for the necklace, and she takes the earrings.
After the poker game, Ben runs down the stairs to catch up to Andie. As Andie walks off, she waves him off with her right hand during the front shot. But in the very next shot, it's her left arm that's seen to be waving.
When Andie leaves Ben's apartment after their lobster date, she forgets her bag at the apartment, but still gets into a cab. Nothing was mentioned about how she ended up paying the cab driver. And the bag magically appears in Ben's office the next day as he and his friends are walking into the office.
When Andie enters Ben's apartment for the first time, and comments on how nice it is, the panning shot of the apartment shows daytime outside. They were just outside and it was night time (after drinks and dinner).
For the NBA Finals to reach 7 games, it would take at least two weeks. All 7 games wouldn't be played in the span of 12 days as in the movie.
Ben plans the date for Game 7 for "Day 13" and the couple go to Game 1 on "Day 2"
NBA Playoffs are usually played only two or three days apart. Ben has tickets for Game Seven for "the 13th day" on "9th day" but before Game Six no one would know whether there will be one, so Game Six must have been played already on "the 8th day", five days before Game Seven.
When Andie goes over to Ben's house for dinner and finds he made lamb she uses the excuse that she doesn't eat meat to ruin the dinner and then takes him to a vegan restaurant. However, on the first night they met they went for dinner and had lobster. This is actually very common, as many people follow a "pescetarian" diet, that is, eating only fish or seafood and not meat, especially red meat such as lamb. Andie could very easily claim to eat only seafood, and therefore act repulsed by the lamb.