Factual error: After all the running about at the end in the snow, no one's breath seems to be condensing.
Continuity mistake: After Bridget has given the appalling introduction to Mr. Fitzherbet, he comes on stage with the author. Mr. Fitzherbet says "I'll just switch this on" and turns the microphone on, but his voice is being transmitted over the mike before he flicks the switch.
Continuity mistake: When Bridget is searching in the fridge for tuna, she wipes her face, and really big smears of her cooking spread across her cheeks. Then when she goes to answer the phone, the smears are practically gone.
Continuity mistake: After Bridget has chased Mark Darcy in the snow, when the scene initially focuses on Bridget, she has some large concrete barriers behind her, they suddenly disappear from the scene after awhile, just before the big kiss.
Continuity mistake: In the shot where Bridget goes to see her mom when the mother is selling the egg separator, her sweatshirt strings are crossed, when the shot cuts to her again they are crossed the other way. After the demonstration, when she is talking with her mother, they are back the same way again.
Factual error: Bridget gets a screen notice ('One new email message') from MSN Hotmail. Hotmail does not give screen notices except for notices through MSN Messenger (which is not what Bridget gets).
Continuity mistake: When Bridget and her parents are driving to the Darcys' house, you can see that the road already has fresh tyre tracks on it. But once Bridget and her dad switch places, the road ahead hasn't got any tyre tracks.
Continuity mistake: When Mark and Daniel are having their big fight, Mark punches Daniel really hard, causing him to fall to the ground. In the shot of the punch Daniel falls face down towards the ground, however in the next shot he lands flat on his back. It would've been impossible for him to have turned around-especially being knocked out.
Continuity mistake: When Bridget's boss at the TV show tells her he wants her to cover the Kafir/Heeney case (the freedom fighter), Bridget is playing with her pen over her right shoulder. The shot then shows her boss is talking to her, but neither her hand nor pen are up - in the next angle of her, she is still waving her pen up by her right shoulder.
Continuity mistake: When Bridget is at the "tarts and vicars" party, her "uncle" Geoffrey pinches her bunny tail for the second time - when he bends over to pinch it he has a straw (or maybe a cigarette?) in his mouth. Just as he does this, Bridget's mother comes along and slaps his hand, he pulls up and there is nothing in his mouth.
Deliberate mistake: When Mark is catching a cab outside "JFK" airport, the cars are driving on the left hand side of the road - if they were in America they would be going the wrong way (the director points out in the commentary that this shot was filmed in London to save money).
Audio problem: When the men are fighting in the street, Bridget and her friends are talking about who to root for, and Bridget's mouth doesn't match what she is saying at all.
Continuity mistake: When Bridget is writing in her diary about the kind of man she must not go for, she is writing on the left page. When the voice over says "And especially must not fantasize about particular person who embodies all these things" there is a close up and she's writing on the right page. Seeing as her diary has a page per entry, not really probable that this would happen.
Other mistake: The last name in the credits from the cast is called Melinda, when in actual fact, the character in the film is called Melanie.
Factual error: After Bridget arrives back in London from a visit to her parents, the camera pans away to reveal the train on which she is supposed to have travelled. You can tell from its distinctive yellow and white exterior that it's a Connex train, but these only run on the suburban commuter routes through south London to East Sussex, Surrey and Kent - nowhere near Mr and Mrs Jones's Cotswold home. Rail services from the Cotswolds are operated by First Great Western (grey high speed trains) and Thames Trains (navy blue exteriors) and terminate at Paddington in west London.
Continuity mistake: Towards the end when Mark Darcy finds her diary, you clearly see the number 29 as he pulls it from under the magazines, then when you see the diary it's open on pages 10 and 11.
Other mistake: When the two gents are fighting in the restaurant Hugh Grant tackles the guy through the plate glass window and they land on the street. Two points: 1) How do they not receive any cuts from landing on shattered plate glass. 2) Check out the glass laying on the street. It is cloudy, a main characteristic of breakaway stunt glass.
Continuity mistake: At the Darcy's party, near the end of the movie, Bridget's hair changes constantly: smooth to slightly mussed, barrette visible then covered with hair, and the location of her part shifting a bit.
Factual error: One that only Londoners would notice or care about, but anyway: Bridget's flat is supposed to be in the trendy bit of Southwark, near Borough Market, i.e. south of the river. Right at the end, when she pursues Mark Darcy, the shop where she catches up with him is clearly in the City of London (the very distinctive Corporation of London bollards are all around, and the shop itself is on Threadneedle St, I think) - meaning that he's walked and she's run right across Southwark Bridge and up King William St in about thirty seconds...
Other mistake: When Bridget's friends come over to her home they always need to call up first. She then can buzz them in by pressing a button that unlocks the building's main door. Yet when Bridget first runs from her flat after Mark with just her bare feet you can hear her feet crunching on snow. She then runs back inside to put on her shoes and a sweater. If she really needed to buzz her friends in then there should have been a hallway where she ran first and not the outside directly. [As an addition: when Mark comes to Bridget's door to show her the newspaper article, as he says ...'I may have come at a bad time...' you can see a hallway behind him. When she runs outside at the end of the movie there is, for some reason, a different set outside her doorway. For example, when Bridget opens the door to run after Mark, it looks like she is running into bright sunlight - lighting way too bright for hallway illumination and especially out of sync with the evening sky.]
Answer: Lots of times in book-to-movie situations and even in some remakes, the producers/writers/directors change names or events. It can be because of personal issues, legal ones or it can even be spur of the moment. it can also be unintentional,when they change it without the specific INTENTION to change it. I think in this case it was more likely that the character/or their name didn't have too much of an impact on the story line, so they didn't feel a pressing need to keep it the same.