When Hitch meets Albert, there is a couple sitting at the top of the stairs behind Albert (the man has a blue shopping bag in front of him). When Hitch approaches Albert, they disappear then reappear in the next shot.
When Sara and her friend are dining in the restaurant Rice to Riches, you can clearly hear the clink of cutlery against dishes in the background. However, all the dish ware in Rice to Riches is plastic, and thus would not make such a sound. You can clearly see Sara and her friend eating out of Rice to Riches' iconic plastic bowls with plastic shoehorns.
In the opening narration Hitch says that communication is 60% body language and 30% is your tone meaning 90% of what you're saying "aint coming out of your mouth" However tone refers to the tone of your voice which does in fact come out of your mouth.
After Alex and Sara finish their first date and she gets into the taxi, Alex's shirt gets caught in the taxi door and it rips; it doesn't just slip off of him. When they meet again at the fish restaurant, Sara returns Alex's shirt (it appears to have been dry cleaned) and it is not ripped.
When Hitch suffers from food allergy the right side of his face and his right ear are shown to swell up. This is shown also in the pharmacy scene but when they get outside and he is drinking from the medicine bottle with a straw his swollen ear and reduced puffy face are on his left side.
When Allegra boards her yacht, she removes her shoes and walks up a boarding ramp. After her short conversation with Hitch, Albert steps up onto the deck before attacking Hitch. When Hitch leaves the yacht minutes later, he has to step up to disembark and the boarding ramp is un-attached and lying flat on the dock. For the amount of real time involved, any tide change would be negligible, the boarding ramp would still be in place, and Hitch would walk down it to get on the dock.
When Hitch's jet ski dies, and he tries to get onto Sara's, you can see on her shirt that she has a safety chord attached to her life preserver. In the next shot, when Hitch is climbing between her and the steering column, the chord has vanished.
When Albert and Hitch first meet, Albert spills his soda on the steps. When Hitch sees how pathetic he is and comes over the spill is bigger even though the can had been set back again and didn't continue to spill.
When Hitch kicks Sara off of the Jetski, she falls right next to it. When the camera goes further out, she is quite a bit farther away from the Jetski. When the camera goes back, she's close to it again.
When Albert is trying to catch Allegra's car and keeps running after the taxi and her car alternately, the sun is shining on the street when he goes for the taxi, and there's a shadow over the street when he goes for Allegra's car.
In the opening scene, Hitch explains to the first 3 men he's helping that the third date with a girl is the most important because that's the date where they'll have their first kiss. When he helps Albert win over Allegra, Hitch prepares Albert for the first kiss to be on the second date (their first date is the Sky Studios fashion event, the second is the Knicks game, after which they have their first kiss).
At the Knicks game, the people in the crowd in the background behind Albert and Allegra changes between shots. Also, initially there are two empty seats between Albert and the next fan to his left, then there are three empty seats when a couple sits down beside Albert. Then at the end of the scene, there are no empty seats.
When Albert grabs Hitch on the boat, the shot from behind Albert shows that Hitch's hands are on Albert's shoulders. Then when we see it from the opposite angle, his hands are instantly on Albert's forearms.
When Hitch meets Cressida on the college steps he is wearing his glasses. When the camera is behind Hitch and you see Cressida smiling at him, the glasses have disappeared. In the next shot Hitch is still wearing the glasses and taking them off.
When Sarah is trying to drive the manual transmission car near the end of the film, a shot is shown of her pushing down the gas pedal and the car taking off. However, her left foot is not on the clutch at all. With a manual transmission, the car would have stalled without having the clutch pushed in and then gradually releasing it while pressing the accelerator.
After Hitch's food allergy, he brings home an assortment of coffee and tea for Sara and himself at her apartment. When he hands her the tea, and when they are each drinking their own coffee/tea, it is obvious that the coffee cups have nothing in them. They clearly hold and drink out of empty cups. When he opens the plastic top, it makes a hollow sound - and is obviously empty.
At the end of the movie where Hitch is giving his speech in the middle of the street. Around 1:49:52 in the upper right corner you can see a red tour bus, and the people on top clearly get up abruptly and start pointing at the camera.