The Halloween-themed strips◊ in 1989 depicted Garfield waking up in an abandoned house to learn he's in an alternate reality where he no longer exists. Unable to cope with being alone, he screams out "I don't want to be alone!", and the next thing he knows he's back in his own time again. Some fans speculated that this meant Garfield was really in an abandoned house and slowly starving to death, and imagining future strips in a never-ending state of denial. Jim Davis himself has said that this is false; he had wanted to write a Halloween strip that was actually scary, and decided on what he felt was what people feared the most: true loneliness.
"An imagination is a powerful tool. It can tint memories of the past, shade perceptions of the present, or paint a future so vivid that it can entice... or terrify, all depending upon how we conduct ourselves today..."
One of the scariest things in this series was the exterior of the house; that panel is a masterpiece of utterly-terrifying desolation.
The scene in the first movie where Garfield sneaks into the train controller room in order to get the train Odie is on back to the station. He accidentally rewires the tracks so every single train will crash into another one, and the entire time, he's making snarky comments about it. ("Seattle? I don't care about Seattle." "Sheesh, you sound like my mother.") Fortunately, he presses the "Cancel" button seconds before they crash.
Also Odie being shocked repeatedly with the shock collar.