Beam Me Up, Scotty!: A lot of the quotes attributed to him were either only said in merchandising and book covers, or said once in the strip and forgotten. "I'm not overweight, I'm undertall" was only ever said as such in the first compilation book, Garfield at Large (he later said it to the reader as part of one of his semi-annual National Fat Week strips), while "Big, fat hairy deal," though quite the catchphrase in its day, hasn't been said in at least three decades. Also, "We're bachelors, baby" has only shown up about 10 times.
Keep Circulating the Tapes: Two Garfield books: Babes and Bullets and Garfield: His 9 Lives are out of print, and neither are listed on the Garfield Website's store. Interestingly, the animated specials based on both books are listed on the Garfield Website store.
For some reason, May 2-5, 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).
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".
Preview Piggybacking: Argubly, the only reason anyone saw the first movie when it aired on The Hub was to see the Equestria Girls commercial that premiered on it (the song, the film wasn't out until two years later). This got lampshaded in a comic that showed a screenshot from the movie, Pinkie Pie's family looking at it in disbelief, a screenshot from the commercial, and the family with happy expressions.
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 the June 21, 1978, August 2, 1978, and July 2, 1979 strips, 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. Frank Welker is his most recent replacement.