The 1989 - 2012 couch gag is the tenth couch gag of Season 23.
The Simpson family run in and sit on the couch as usual, but above the couch is a banner that says "America: Most powerful country in the world". They then go through a time warp, starting with 1989, the year the show premiered, to 2012, the year this episode aired in. Every other year, something interesting happens:
In 1989, the Simpson family holds up American flags. In 1991, Leon Kompowsky moonwalks from the middle to the left of the room. In 1993, Stampy is seen with Ralph Wiggum (who is giggling) in his mouth. In 1995, Poochie is seen in the room gesturing. In 1997, O'Reilly the Leprechaun is sitting on the TV while winking at the camera. In 1999, the ghost of Maude Flanders haunts the family. In 2001, Baron Von Kiss-A-Lot is sitting in the room. In 2003, Goose Gladwell is in the room. In 2005, Chazz Busby is in the room. In 2007, Homer is holding Plopper and is tickling him, who squeals in response. In 2009, Princess Penelope rides across the room on a unicorn prop. In 2010, Homer and Bart change the America banner to a sign that says "TOO BIG TO FAIL WE HOPE". Finally, in 2012, Maggie holds up a flag for China.
As the years go by, the character designs in the Simpson family are very noticeable. From 1989 to 1994, they have their design as seen in the Season 1 and 2 couch gags, and from 1995 to 2012, they have their current design.
The "America: Most powerful country in the world" banner has noticeable changes, too; from 1989 to 1992, it looks all new. From 1993 to 1996, it barely starts to fade and has only a couple of rips on the left. From 1997 to 2000, its still barely faded, has more rips and has a hole nearby the second A of "America". From 2001 to 2004, the letters start to fade along with the background, the rips get bigger, and the hole gets bigger, and from 2005 to 2010 (before Homer and Bart change the sign), the letters and background are almost halfway faded, and the rips and hole are much bigger.
From 1989 to 2006, the 4:3 analog TV is seen, and from 2007 to 2012, the 16:9 widescreen TV is seen, even though they didn't switch to that widescreen TV within the show until the couch gags for the last twelve episodes of Season 20.