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.
Flash Harry: I wanted to talk to you about the problems with the last batch.
Tania and Tara: Problems, Flash?
Flash Harry: The slightly bitter aftertaste, the people going blind after the second glass, that lady what died...
Tania: She was old.
Tara: She could have gone at any time.
Flash Harry: She was thirty-eight.
Tania and Tara: Yeah?