When Corina calls Desmond on her original iPhone, her Service Provider shows a 'Tesco' and the time as 13:36. She gets an automated message to say that 'The person you're trying to call is...'. She immediately tries again, this time to leave a message. Her Service Provider is now EE and the time is 16:12.
When Corina and Mel are eating beans on toast, Mel puts a piece of toast and some beans on her fork, raises it to her mouth, stops, doesn't eat it, then lowers the fork, scrapes the food off it, then picks up more toast and beans.
When Corina tries (and fails) to light the cooker, then touches the cold radiator, the film implies that their gas supply has been cut off, but having no gas wouldn't cause an electric ignition on a cooker to stop ticking.
When Corina is cleaning the kitchen in the morning, she tells Mel "I'm still not feeling well, so don't come too close". Literally seconds later, Mel stands right next to her mom and kisses her on the lips.
When Corina returns home from collecting the suitcase, she shuts the door and puts the latch on. A few moments later Mel just walks in. Even if Mel has a key to the door, which she most likely does, the latch still needs to be taken off from the inside...otherwise what's the point of it.