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):
Some useful tips I've stumbled upon and seems no one else has...
The meals are almost always are grouped on to small plastic trays which separate each “course” or meal option (pictured).
If you’re anything like me (or seeming everyone else who sits by me), you might be tempted to “consolidate” all of the wrappers, scraps, and small items which are leftover (napkins, plastic cutlery, salt/sugar sachets, etc.).
While this may make your service tray look tidy, it’s actually very bad for the recycling efforts of the on-board crew. All of these service items are later separated for recycling, and consolidating makes this process more difficult or forces the recyclable materials to be disposed of as general waste (this is especially the case of you stuff things in plastic water bottles).
Next time, embrace a disorderly tray in an effort to aid recycling on flights.