When William is walking down the street during the seasonal montage (during the song - Ain't No Sunshine When She's Gone), his sister has a new, shorter hair style. In her next scene where she gives William the phone numbers, her hair is back to the original. All the next scenes have the new style again.
Honey's hair is definitely a slip up because she comes into the shop right after William has walked through the seasons and has the short style the rest of the film - and even though the walk down the street shows 4 seasons (?) he says in the following scene that he apologizes for his behavior the past 6 months!
The scene where the seasons change is suspposed to represent a whole year going by, so in that time, his sister has different hair-styles. She gets her hair cut, it grows back again. It also shows her fighting with a man, who presumably was meant be a current boyfriend, then she comes into the book shop with his flat-mate. This is not a slipup, merely a ploy to show the passage of time.
How did you notice her hair style changing but not the change of the seasons (sunny, windy, snowy, rainy). I'd be more worried about the instant change of weather than his sister's hair style changing. Her hair was short, I think, when Fall was the season and long again at the end of the scene. People's hair would generally grow back out during that amount of time.
I agree with the other two about the passage of time. Also, if you look closely, you'll see, at the beginning of the montage, a very pregnant woman. Then, towards the end of the song, the same woman is standing there holding a baby. My biggest concern in that scene is Hugh Grant's outfit. Are we to believe that he wore the same clothes everyday?
Okay guys - I got the passage of time, that's why AFTER the passing of time during which she cut her hair, when she comes into the shop AFTER the passage of time she has the SAME hair cut as BEFORE the passage of a year!!