There are 2 Far Side cartoons on the wall by my desk:
Farmer Brown froze in his tracks; the cows stared wide-eyed back at him. Somewhere in the distance, a dog barked. (Cows with a diagram of how to butcher and package a human)
Again the doorbell chimed. With his wife out of town, and not expecting any visitors, Mohammed began to grow uneasy. (A mountain outside a house)
Street corner at center. On one side, a nerd on roller-skates grooving to a Walkman, skating downhill towards the intersection. On the other, obscured from the nerd’s view and bound for the same intersection, Tarzan on elephant-back leading a charging herd of elephants.
“Brian has a rendezvous with destiny.”
One of my favorite is the time the Far Side caption accidentally got switched in the comics page with the Dennis the Menace caption - improving the latter strip no end.
The Dennis the Menace cartoon has Dennis looking at someone earnestly and saying something like “I see your tiny, petrified skull sitting labelled on a shelf”. 
[The Far Side was of a Neandethal fortune-teller. Can’t remember the Dennis caption that went with it]
“Late at night, Dave’s own stomach would foil his attempts at dieting”. Picture is of a disembodied stomach standing in front of an open refrigerator while it’s owner sleeps in the background.
An upset-looking cat with 2 wooden legs broods in the corner of a pet store. A glass bowl with a mean-looking fish in the foreground. Label on the bowl: Piranha, $299. No caption necessary
A bunch of soldiers are storming a castle. One soldier points down at the moat at says “Oooo look! Goldfish!”
“Testing to see if laugher really is the best medicine”. A bunch of doctors pointing at laughing at a patient in a bed. Gets me every time! 
A couple of guys and a couple of elephants are in a bar. One of the elephants and one of the men seem about to fight. The other elephant tries to hold back his friend, saying “Cool it Marv, he probably didn’t realize you were an elephant when he told that joke!”
Frame 1: A guy standing by himself.
Frame 2: A flying saucer lands in front of him.
Frame 3: An alien from the saucer punches him in the nose.
Frame 4: Saucer flies away.
Caption: Bob never knew what hit him.
This one wrecked both me and my father-in-law when we read it in The Complete Far Side, which my lovely wife bought me for Christmas one year:
The picture is an outdoor scene, sun just coming up over the hilly horizon. In the foreground is a tree. Next to the tree, flat on his stomach, is an ape-like creature. He’s clearly just fallen from the tree, and the expression on his face is one of stunned amazement.
The caption reads, “The Dawn of Man.”
A woman shouts out of her living room window to her dog thats barreling toward the pet-door (that’s been nailed shut on the inside).
“Here Fiffy! Faster Fiffy!”
Scene: a lab. All the scientists are yelling at each other and arguing. Caption: Too many scientists; not enough hunchbacks.
Scene: a lab. Equations on the board, microscope, etc. An ice cream truck can be seen outside the window. Scientists, with expressions of glee, are running outside!
Scene: construction site. A skyscraper has fallen over and is perched on the head of a construction worker. The foreman is saying “And THAT is why we wear the hardhats.”
That reminds me of another one. Three humans and a dog playing poker. “Fold.” “Fold.” “I fold.” Dog, tail a-wagging: “Dangit, how can they tell!?”
My favorite work-related one: “Early microbiologists” - cavemen sitting around benches with Petri dishes. They’re squinting at them really hard.
Favorite non-work-related one: A bunch of vultures are gathered around, eating. One of them is wearing a cowboy hat and coat. He says, “Hey, look at me! I’m a cowboy! Howdy, howdy, howdy!” It’s the "howdy"s that make it.
In one of the anthologies, Larson gave another example of the same thing happening. In that one, Dennis the Menace features the family sitting down to dinner with the caption of Dennis saying “Oh boy! Hamsters again!”
That made me laugh out loud.
From the “Censored Cartoons” section of the collection Prehistory of the Far Side:
A python or other large snake in a baby’s playpen, its head outside the bars, with a nonplussed expression as it realizes the huge bulge in its middle now prevents it from getting its body out through the bars the way it (presumably) got in.
No caption, as such, only author’s commentary: “No, you didn’t see this one. Turn the page.”
A flying saucer on landing struts, resting on the ground in front of a staring crowd of humans, with an open hatch and a ramp going down to the ground. An ET sprawled on the ground at the bottom of the ramp, apparently having stumbled and tumbled down it. Several more ETs visible around the open hatch, one glaring down at the tumbled one with a disgusted expression.
“Beautiful! Just beautiful! So much for instilling them with a sense of awe!”
A brontosaurus* peacefully browsing. Time machine in the background. Scientist walking up behind the brontosaurus with what appears to be a giant thermometer.
“Moments later Dr. Farnsworth’s career came to an abrupt end, leaving the warm-blooded/cold-blooded dinosaur debate undecided.”
*Apatasaurus, whatever.
One of my faves:
A deer is standing on a stage, eyes as big as saucers. The caption: “Carl finally got his big chance to perform, but when the floodlights came on, he froze.”
A dog is standing outside the front door with a hose in his mouth. His wife is in the doorway: “So. . .planning to roam the neighborhood with some of your buddies today?”
Man sitting on public toilet looking at empty toilet-paper spindle, and at a strip of something trailing into his stall, from under the partition. Visible (at least to the reader): Said strip is trailing from a wrapped foot in the next stall over.
“It was an honest mistake. Nevertheless, in moments Harold would find himself bearing the full brunt of the Mummy’s wrath.”
Similar: Next to a motorboat, a guy is floating in the water, holding a tow-rope, tips of water-skis showing, OMG look on face. On the other side of the boat two guys stare down at the water; one points.
“Look, Sid, was I lying?! That sucker’s longer than the boat!”
:o Beaten to it by Sailboat. Sick minds think alike.