Sliding Doors

Helen Quilley, played by Gwyneth Paltrow, just got fired. And now she missed the tube train, too. Some kid was in her way, slowing her down for a second.

But wait! She didn't miss the tube! The mother of the kid pulled the kid away, so Helen was never slowed down.

The Helen who got on the tube meets an interesting guy on the tube, and when she gets home she catches her boyfriend cheating. She breaks up and gets an Important Haircut, turning blonde.

The Helen who missed the tube gets robbed, and comes home to be comforted by a boyfriend who had more time to cover his tracks a bit. She keeps her old hairstyle and remains brunette.

As time goes by, blonde Helen and brunette Helen live increasingly different lives. And yet they seem to somehow remember each other, each of them getting a sense of déjà vu when encountering something that is important in the counterpart's life.

Tropes used in Sliding Doors include:

Alternate Self: Blonde Helen and brunette Helen.
Alternate Timeline: Two time lines - one where Helen got to the tube and one where she didn't.
Bitch Alert: Lydia.
British Accents
Expository Hairstyle Change: Without the difference in haircut, it would be hard to tell the two time lines apart.
Flash Sideways: Each Helen instinctively picks up on things important to the other Helen.
Flatline: Used to show when one of the characters dies in one of the timeliness.
Foreign Remake: The storyline recycles in a Lighter and Fluffier way the plot of an earlier film, Blind Chance.
For Want of a Nail: A kid getting in her way for a second, making the difference between two radically different lives.
Girlish Pigtails: Helen, when she has to work as a sandwich delivery girl and waitress.
Hard-Drinking Party Girl: Anna. Gerry nearly gets away with the extra wine glass in the laundry basket (left by Lydia) suggesting Anna left it there at one of their house parties. Anna admits it's something she'd probably do.
Important Haircut: The change in hairstyle signifies a change in lifestyle.
In Spite of a Nail: Helen met James. Helen lost her unborn child. Gerry has impregnated Lydia. Helen has found out about Gerry and Lydia and has left Gerry. Which time line am I talking about? It's worth noting that all of those things come true in very different ways and at different times, however.
Ironic Echo: In the first time line when James and Helen meet he says "Cheer up, you know what the Monty Python boys say", and Helen replies "Always look on the bright side?". James then corrects her with "No one expects the Spanish Inquisition". When they meet again in the next time line he says the same thing and Helen responds with the latter line this time.
Life Will Kill You: One of the main characters is just standing there, having what would have perhaps been the most important conversation in a long and happy life. Suddenly a car runs over her. Downer Ending in one time line, but it is indicated that the trauma of her own death helps her to get a happy ending in the other time line.
The London Underground: The plot diverges at the main character catching / missing her train at Embankment station, setting off events for the rest of the film (which shows us two parallel lives from that point on). The actual scenes underground were filmed on the Waterloo and City Line.
Love Transcends Spacetime: Two time lines, two lives, one romance.
Manic Pixie Dream Girl: Inverted. James is the Manic Pixie Dream Guy for Helen.
Mirror Monologue:
- Lampshaded after Gerry almost had his "secret romance" revealed to his live-in girlfriend:

Gerry [looking in the mirror]: You have two head problems. One, that was close, very close. Put in layman's terms, she nearly caught you. Two, and this is far more worrying than the first one, you're talking to yourself in the mirror again. Really bad sign.

- Later brought up again by his friend:

Russell: You've been talking to yourself in the mirror again, haven't you?

Missed the Bus: The plot is based on the difference it makes to this woman's life whether she gets or misses a tube train.
Service Sector Stereotypes: The worse-off scenario had the heroine fired from her job to become a waitress / delivery girl for a sandwich bar, much to the delight of her boyfriend's "other woman", who revels in exploiting and insulting her, knowing that her rival can't fight back.
Truth in Television: As many people who have ridden the tube can testify, a single second can make all the difference between catching and missing a train. The tube keeps London moving, and it's not going to stop for you.
We All Live in America: A small case but Helen and Anna reference Jeopardy, an American game show which does air on some channels in the UK but is unlikely to be talked about casually by two British women.