Nica Lorber singing "Yellow Submarine" in Flaky's voice during a behind the scenes special.
From the same special, "Lippy" Lipman sucking his thumb while demonstrating his Nutty voice.
From "Blind Date." Mole feels (dead) Lumpy's antler touch him and takes it to mean that he's being cuddled. He hugs Lumpy just as a bird nips at Lumpy's exposed brain, causing Lumpy's arm to smack him.
In "Idol Curiosity", an ever-growing crack in the ground takes a taxi to catch Sniffles. You can even see the crack toss a few coins to the taxi driver!
Flaky screaming at the sight of baby ducks in "A to Zoo", Lumpy's unintentional massacre of the baby chicks and Flaky's shell-shocked expression.
From that same episode, Cuddles' quest to jab every animal he finds with a stick.
Flippy riding an unicorn. A pink, winged unicorn on a flower field, while he's smiling happily.
Lumpy saying "Merry Christmas!" at Petunia's birthday party at the end of You're Baking Me Crazy.
Any time The Mole does something stupid that blind people shouldn't be doing, such as driving and breaking the garage door.
Pretty much any scene with The Mole in A Sight For Sore Eyes, such as driving out of his garage before the door even goes up AND walking down the middle of the road with a lawn mower.
Also walking into an open manhole in Who's To Flame, and taking pictures in See What Develops.
Hear Today Gone Tomorrow features Lumpy losing his hearing. So what next? He comes home, not hearing the alarm, and the other objects in the house proceeds to beep very loudly, but since he doesn't hear anything, it doesn't bother him. Over at Flippy's house, he wakes up from his deep slumber and flips out the next second. He then goes over to Lumpy's house, equipped with an axe and rings the door bell. Lumpy, in the middle of baking cake, doesn't hear anything. Fliqpy gets frustrated and throws the axe on the ground. The rest of the episode is him trying to kill Lumpy, but fails repeatedly. Best part is, it's the first time he never kills anyone while flipped out.
In "Sea What I Found," Lumpy looks out the window, sees moving water and gets sick. The next shot reveals that the boat was still in the dock, and what he actually saw was a washing machine Russell was towing.
From "Junk in the Trunk." Lumpy sees three options for transportation that he can use to pursue Lifty and Shifty: a racecar, a motorcycle and a pogo stick. Which does he choose? The pogo stick, of course!
"Remains To Be Seen", once you get past the horror, has Lumpy having to choose between many weapons in a small room to defend himself against his Zombie Friends. We see a lot of weapons that seem plausible, including a chainsaw... only for him to stick a leaf blower onto his arm and wave it with triumph at Flippy. Fliqpy's face seals the deal.
In one scene in "Double Whammy(Part 2)," Cuddles, Toothy and Cro-Marmot are playing dress-up.
When they come out of the closet, Cro-Marmot has a dress stretched over the ice block that encases him.
Happy Tree Friends hate mail, as read by the cast, studio staff, and various people on the street.
The line "I wish I had a scrumptious lollipop" sounds funny on its own. So what happens when Nutty says it? Laugh all the way to a candy shop in your vicinity!
The scene from "Ski Patrol" where Lumpy is receiving instructions to put chapstick on Giggles. Midway through, the film skips ahead to Lumpy performing open-heart surgery on Giggles, which Lumpy doesn't realize from his perspective. The film skips ahead again as Giggles stands comfortably, then keels over as her chest erupts with blood and various objects, including a shoe.
Asbestos As I Can Do, which starts with a "previously on" segment which is just about a minute or so of gruesome deaths from other episodes. Then we get to the episode... Which is Lumpy knitting an antler warmer. It then does a "next time on" segment which is more gruesome deaths from other episodes.