We see Maureen's bare legs as she sits on the side of her bed and puts on a pair of slippers. In the next shot she stands up, wearing dark blue tights or long socks.
When Martin picks up J.J. with his van, it is heavily raining. A reverse shot shows J.J. from the right, now it's only slightly drizzling.
When JJ and Jess go to her special spot near where her sister went missing, they watch some swifts flying above them. Swifts are migratory and winter in the tropics and would not be flying in the UK between January and February when the film is set.
The suicide pact made between the characters is established for period New Year's Eve - St Valentine's Day. When Martin and Jess meet her father opposite the Houses of Parliament, the clock on the tower of Big Ben shows 4.40pm. At that time of year it would already be dark but it is bright and sunny.
When Martin is on the Tower, sirens are heard twice and they sound like American sirens not European ones that one would hear in London.
When the four are standing on a rooftop in February their breaths can't be seen in the air. This reveals it's in fact a set, shot in room-temperature.