Other mistake: After Kelly and the twins climb out of the sewer tunnel and hear footsteps, the twins sit atop the steel grating pretending to cry while the rest hide below. All the walls and paintings behind the twins are the very same ones behind Kelly, Taylor and Andrea later, when they've finally reached their target painting, in a different area of the gallery. We can even see the square of wood planks covering the area of the steel grating seen earlier.
Other mistake: Just before the Cheltenham bus arrives before the hockey game, Geoffrey Thwaites is reading The Sun. In the close-up the headline reads "Shot in the arm, new Minister for Education announced today", with a photo of Geoffrey beside it. However, the actual article has nothing to do with Geoffrey. It is about Andy Millman's temper, and how he verbally abused a Downs Syndrome child. Andy Millman is a fictional TV character played by Ricky Gervais in "Extras". The other two articles make no sense as well.