During the final act, Willoughby (Asa Butterfield) takes Don's (Finn Cole) tie to face one of the creatures allowing the others to escape. However in the next shot when Don is talking to Clemsie (Hermione Corfield), his tie is back around his neck tied neatly. When he returns to Willoughby, tie is gone again.
In one brief scene, Don is sitting in a second floor classroom looking out a window at Clemsie, who is standing on a paved path several yards from the classroom building. You can tell by the light shining on Don's face and from Clemsie's shadow that the sun is shining against the window. Clemsie appears to see Don wave slightly at her, but in those light conditions, all anyone could see in such a window would be glaring light. You'd have to stand at the window and create some shade to see through the glass well enough to perceive anything meaningful.