Revealing mistake: Ollie throws a flower pot at Stan. Stan ducks and the pot continues through the house, breaking a window and hitting a gardener outside. On the DVD of the film, the long wire the flower pot is attached to is visible.
When Oliver pulls down the chimney pipe from the stove, he is hit by a cloud of soot. At this point, you can see that the top of Oliver's hat is clean, but when Stan appears in the next shot, his hat is now covered in soot.