Katherine Heigl's hair color seems to change from scene to scene. When she's confronted by her sister after the article in published it is brown/red. Then blonde at work the next day, then brown/red at the bridal shop.
George is said to have dropped out of college when Jane is telling Tess about him but later at the engagement dinner during the slide show, George has doctoral regalia in one of the photos.
In one of the scenes where Jane is sitting in her office at work, her ponytail is curly. Once George summons her to his office, her ponytail is straight in a matter of seconds - noticeably straight.
When Kevin is eating breakfast with Jane he is scruffy, but when he walks out of the restaurant he is clean shaven.
When Casey, Jane and George are walking down the hall at the office - Casey (Judy Greer) has a purple sash as a belt that is clearly tied at her right side. When Tess and Jane are walking out of the building, the sash/belt is directly in the middle of her stomach.
During the rehearsal dinner slide show a picture is shown of George at a graduation. The regalia is a mix of undergraduate (the flat mortar board) and post graduate (the bars on the bell sleeves of the gown). Earlier when Jane is telling Tess about George, Jane states that George dropped out of college.
Jane's blouse in her bridesmaid sari is backwards. This could be because she got dressed in the cab by herself (there was no-one around to correct her) and she's also not of Indian origin. The sari being on backwards could be an intentional error by the film-makers.
During the end credits, the camera pans over text that reads "Mrs. Doyle had 27 bridesmaids, all dressed in the dresses she had collected over the years." Jane actually had 29 bridesmaids when you also count her sister and Casey (her friend). However, Tess and Casey are not wearing Jane's old bridesmaids dresses, and therefore don't count toward the total who are wearing the dresses. They are also maids of honor rather than just regular bridesmaids.
Even though its raining very heavily, the bar becomes more populated as the scene progresses. Most people wouldn't venture out in such a storm to go to the bar, but those already out who got caught in the storm likely might have sought shelter in the bar while waiting out the storm.
After Jane, puts away her 2 latest bridesmaid dresses, she sits down to read the Commitments section of the paper. The article about the "Carnival Wedding" states that the bride has flowing blond hair, but in the picture she is obviously a brunette.
In the beginning of the movie when Jane and Casey arrive at the office and Jane asks Gina a couple questions, the same shot of Krysten Ritter shaking her head and saying "no" is used twice.
When driving through the rain, the water is running down the front window. If they actually were driving, the water would go back towards the roof because of the wind and speed of the car, and not running down the front window.
When Kevin is flipping through the photos of the dresses on the back of his camera the rate at which the image change does not match the thumb pressing the button.
In the scene where the sister is flipping through the newspaper clips, the article text doesn't match the headline (it's some completely unrelated article, with a wedding headline pasted on).
Right before Jane realizes she is yelling next to a 50th anniversary party, she is smacking her clutch purse against the wall in anger. Not only does her mouth not move as you hear her screaming, "Why, why, why?" but the sound of the purse hitting the wall does not match the points in which it hits. The last two smacking sounds occur while she holds the purse in the air and then after she has hit the wall and now has the purse by her side.
When the plane carrying Tess lands in New York, the shot includes a couple of palm trees in the background haze (not geographically correct for a New York City airport).
The day after the rehearsal dinner, which would have been Tess and George's wedding day, George calls Jane and asks her to be his date for a benefit where he has to give a speech. As if George would have agreed to speak at a benefit that was the same day as his wedding day.
When Jane and Kevin go to the shop to register gifts for George and Tess, Jane explains that she is selecting a casserole dish so that Tess can make her mother's Christmas roast, but a major plot element is that George is a vegetarian and Tess is pretending to be one too.
When Jane attempts to get her life in order, she empties her closet and throws out all 27 bridesmaid dresses in garbage bags. Yet at Jane's wedding, all 27 former brides are wearing their wedding's bridesmaid dress.