The episode is Season 4, Episode 16, titled "Duffless".
The slip-up occurs when Homer is being sentenced for DWI, just as the judge's gavel comes down around 9 minutes into the episode (the next frame is Patty/Selma stamping a "VOID" stamp over it):
You can see Homer's address (sorry for the bad screen capture, but it's the best I could do), and the State is labeled with the abbreviation "NT"... perhaps a reference to the Northwest Territories in Canada? The Zip Code appears to be 48007, which is actually Troy, Michigan.
Through years and years of watching the Simpsons, I loved to use a TiVo or DVR to pause the screen just as they flashed through different scenes. You can usually catch an off-handed joke or comment, but this is the first time I've ever seen a State referenced in the history of the show!
Enjoy!
