I segreti del settimo piano

In minute 44 "Sebastián" stands before a front door of one of the apartments he assumes his children are being held captive inside. To the left of the door is a white wall. After he opened this door the camera switched position, now showing that door from the other side. Unexpectedly there is now a window with a curtain where actually should be a wall.

So, the inside of that apartment has been shot elsewhere.