Bart Carny - S9-E12
Corrected entry: The Simpsons arrive back at their house and discover that the 2 carnival workers they let live with them have boarded up every window and way into the house. However, when they are in the tree-house, you can see the windows on the upper-floor have remained un-boarded.
Correction: We never actually see the back of the house when the Simpsons arrive so we can't safely assume that it's all boarded up. More importantly perhaps, we don't see the back of the house until several hours later, so it's quite possible they simply took the boards off.
David Mercier