Kirk Kettner (Jay Baruchel) is a twenty-something TSA officer employed at the Pittsburgh International Airport along with his friends, fellow TSA officer Stainer (T. J. Miller), airline reservations agent Devon (Nate Torrence), and baggage handler Jack (Mike Vogel). Kirk has a poor track record with dating and is hoping to reconcile with his self-centered ex-girlfriend, Marnie (Lindsay Sloane), who despite having broken up with him two years earlier, and having since found a new boyfriend Ron (Hayes MacArthur), has remained close with Kirk's parents (Debra Jo Rupp and Adam LeFevre), brother Dylan (Kyle Bornheimer), and pregnant sister-in-law-to-be Debbie (Jessica St. Clair).
At work one morning, an attractive young woman, Molly McCleish (Alice Eve), arrives at the passenger terminal to board a flight to New York City. While proceeding through the TSA security checkpoint, Molly's striking looks attract unwanted attention from several male TSA officers who try flirting with her awkwardly. Kirk is the only TSA officer to treat Molly courteously. On the airplane, she realizes that she accidentally left her cellphone in the airport security area. When she calls up her phone, Kirk answers and arranges a time to meet the following evening so that Kirk can return it.
When Devon and Kirk arrive at the Andy Warhol Museum, where Molly, a lawyer-turned-event planner, is managing an event, Kirk collides with Molly's sister, Katie (Kim Shaw) and spills his drink on the museum director. Kirk takes the blame for the incident to protect Katie, after which a grateful Molly offers Kirk tickets to a Pittsburgh Penguins hockey game at the Mellon Arena. When Kirk and Stainer meet Molly and her best friend Patty (Krysten Ritter), who develops an immediate mutual loathing with Stainer (he subsequently refers to her as "The Hamburglar"), at the game, Kirk, still convinced Molly is not interested in him, assumes Molly meant to set him up with Patty, until Patty explicitly tells him of Molly's interest.
The two begin to date after this, with Kirk confiding in her his dream of becoming a pilot someday, though Stainer predicts their relationship will fail as he deems Molly a "10" in a scale of attractiveness, and Kirk only a "5", telling him a girl he loved once broke up with him for this very reason. Patty, for her part, believes Molly had only chosen Kirk because he was a "safe" choice after being hurt by her last boyfriend, Air Force pilot Cam (Geoff Stults), who assumes Kirk is a waiter and attempts to order drinks from Kirk when they first meet, and then believing Kirk to be a homosexual friend of Molly's.
Molly then invites herself to Kirk's family lunch, where she charms his family and even Ron after highly intimidating the men of the house with her looks. Molly's attentions to Kirk stir jealousy in Marnie, who feels upstaged by Molly's attractiveness, and takes a sudden interest in Kirk again.
After returning to Molly's apartment, Kirk ejaculates prematurely in his pants when things start to heat up, just as Molly's parents (played by Eve's real-life parents, Sharon Maughan and Trevor Eve) arrive for a surprise visit. Desperate to conceal the stain on his pants, Kirk seems discourteous by avoiding to stand up and shake hands, and quickly leaves Molly's apartment. Molly grows cool to Kirk after this, believing he fled to avoid meeting her parents. At Jack's urging, Kirk admits the true reasons for his leaving, and their relationship resumes.
During a date, Kirk suggests to Molly that she throw a birthday party for Katie (with music provided by Stainer's Hall & Oates tribute band, "Adult Education"). Kirk is troubled, when Molly is intentionally vague about Kirk's line of work to her parents. To add to his troubles, Molly's macho ex-boyfriend Cam shows up and messes with Kirk by deliberately alluding to Molly having some sort of "defect".
After the party, both of them go back to Molly's apartment and make out, where Kirk discovers Molly's "defect" is slightly webbed toes, which Kirk considers so minor that he decides that she is indeed too perfect for him. Molly is upset that Kirk is so insecure that he felt he could only be with her if something was wrong with her. After telling Kirk that she and Cam had broken up because of his own insecurities, with him even cheating on her, she admits she had indeed asked Kirk out because she considered him safe and breaks up with him.
Kirk leaves and later resumes his relationship with Marnie, planning on a family trip to Branson.
Stainer and Patty realize their mistake in telling Kirk and Molly it wouldn't work out; Stainer tells Kirk that he is a "10" too. They pull Kirk off his plane as the aircraft prepares to depart to Branson as he tries to leave with his family and Marnie, while Patty brings Molly to the airport. Kirk rejects Marnie during an unorthodox airport pursuit and meets Molly in the airport where she tells him that he is out of shape and she asked him out because she thought he was safe and wouldn't hurt her. She then continues to tell him that she doesn't care what he is employed as and that she misses him and wants to be together with him. Kirk and Molly then make up and resume their relationship, even if their friends don't approve it.
Later, as a surprise, Kirk is seen walking on the airport Tarmac with Molly where he takes Molly on a trip in a small plane, with him as the pilot. The couple are last seen happily together in a small plane taking off from Pittsburgh airport.

Question: What mistake did Staine and Patty realize they made ?
Answer: They told Kirk that he and Molly wouldn't last.
Alternative answer: Telling Kirk and Molly that they would't work out