Missing Episode: For some reason, May 2, May 3, May 4 and May 5, 1990 never made it into Garfield Takes Up Space, and the next book (Garfield Says a Mouthful) starts with the May 6. This is true of the "Fat Cat 3-Pack" reprints of those books, but they finally return in the colorized "square" reprint of Takes Up Space (they're also available on garfield.com).
Recycled Script: Some gags have been repeated in the strip's 30+ year history.
For a less recent, but more exact example, December 3, 2001◊ was recycled into December 7, 2002◊. That's right, only a full year later.
August 16, 1985◊ (a daily strip) was recycled into July 19, 1998◊ (a Sunday strip), with a secondary punchline added. It was also used as a US Acres strip here◊.
Take June 23, 1993◊, subtract the neighbor's mail, add Garfield looking into the wallet, and you'll get January 1, 2001◊. Happy New Year.
Not as big as the above examples, but it seems rather odd that the strips published on June 7 and June 29 of 1991 (barely three weeks apart) would both have the same basic gag.
A cross-media example: The Garfield Show episode "Fame Fatale" and Garfield: A Tail of Two Kitties have the same basic plot of Garfield switching lives with a British doppelganger.
An Animated Adaptation example: The Garfield and Friends episode The Legend Of The Lake has a throwaway gag where Garfield says "This is a statue honoring Buttons, who set the world record for horse swallowing when he swallowed one horse. Tied for second place is everybody else in the entire world." Decades later, in the The Garfield Show episode Up A Tree, a Kent Brockman News reporter makes the exact same joke, but with "hippopotamus biting" instead of "horse swallowing".
Screwed by the Lawyers: There was a short run of Believe it, or don't gags... until PAWS Inc. got a cease-and-desist letter from the Robert Ripley estate
Sandy Kenyon voiced Jon Arbuckle in Here Comes Garfield, but all other traditionally animated Garfield specials, including Garfield and Friends, used Thom Huge as Jon's voice actor. Then Breckin Meyer played him in the live-action moviesGarfield and Garfield: A Tail of Two Kitties. After that, Garfield Gets Real, Garfield's Fun Fest, Garfield's Pet Force and The Garfield Show utilized Wally Wingert's voice for Jon.
In the very first animation, a nameless short film from 1980 that was just animated versions of thesethreestrips, Garfield was voiced by Scott Beach. Starting with Here Comes Garfield, Lorenzo Music was his voice actor for all the animated adaptations. After he died, Garfield has been voiced by several other voice actors, most notably Bill Murray in The Movie (which is either a Casting Gag or a coincidence, considering who played Peter Venkman in both Ghostbusters and its cartoon show). Frank Welker is his most recent replacement..