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.
At Pedro's baseball game when Tess is pretending to be vegetarian to impress George, she agrees with George in that they would never be vegan because being vegetarian is hard enough. When Jane calls her out on lying later, Tess says she could like soy milk if she tried. Soy milk is a vegan product. Vegetarians can still drink regular milk.
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.
When Jane and Kevin are in her living room after showing him all 27 of her dresses, Jane is holding a glass of water in her right hand, with both her feet on the table. She then puts her feet back on the ground and places the glass on the table. In the next shot, Jane is seen with her feet on the table once again, the glass of water still in her right hand.
When Jane steps outside her friend's engagement party to yell in anger, the shot shows the back of her as she screams facing a dark wall. The shot returns to one of her face as she is yelling, when a cough is heard from the 50th anniversary party going on in a large room where the dark wall once was. The black wall has disappeared and a party of mostly white decorations is now in it's place. Also, the door behind her closes without anyone closing it. (The door behind her when she is looking at the anniversary party is not actually the door that she came out of the club from, it is a door off to the side. The door she comes out of is a push bar door, not a door with a doorknob.)
In the beginning of the movie when Jane is pushed out of the way during the bouquet toss, she is wearing her floral head wreath as all the other bridesmaids are. In the next shot when Kevin is helping her up, the wreath is not in her hair, nor is it anywhere on the floor as if it fell out when she fell.
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.
After Jane gets hit in the head while trying to catch the bouquet at the wedding, she has what appears to be a black eye while sharing the cab with Kevin. When she gets out of the cab and is speaking with Kevin, the black eye has disappeared.
After Jane is confronted by Tess, Tess answers the phone to Kevin and then throws the phone onto the couch. After Tess shouts "bridezilla's on the loose" the camera moves back to Jane and the phone is no longer on the couch.
When Jane is going into her father's shop Tess is right behind her, then after a cut to Jane's Father there is a cut to the door where Tess is seen running from the side before getting the door in her face.
When Jane is wearing one gown (from Suzanne and Greg's wedding), while carrying another in a bag, she hails a taxi and affixed on its roof there is an illuminated advertisement panel and its number is "9Z66". However, in the next two shots the ad panel has vanished and the taxi's number is now "8Z30". (There is a missing shot which can be seen in the deleted scenes.)
During the breakfast scene when Tess and Jane's father is serving them mickey mouse pancakes, Jane is holding a coffee cup. When Tess gets up to answer her cell phone Jane is shown from the back holding a utensil. When the camera goes back to a front view of Jane, she is holding the coffee cup again.
All the computers shown in the movie are Dell, which would run Windows. However, when Tess is flipping through her office looking for her lost calendar, the Dashboard from the Mac OS pops up on the screen.
When Jane & Kevin first go into the bar to get out of the rain, they are both soaking wet. Just a few minutes later when they get up to sing, they are mostly dry. Even in a hot, dry bar, they couldn't have dried out that fast.
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.
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.
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.
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.