Li'l Abner, with its cast of yokels and gullible city-slickers, was quite fond of the trope. In one particularly notorious example, Abner himself was sucked into a machine designed to capture him simply by writing "Don't look in here" on a face-sized hole in its side.
Abner: It says, "Don't look in here!" I wonder whuffo'?"
Calvin attempts to lure Susie to a spot where he can hit her with a water balloon, but she doesn't notice him dropping (planting) his letter with an obvious secret code until he loudly exclaims to Hobbes that, "I hope Susie doesn't read our secret letter, because then all our plans would be ruined." At which point it backfires spectacularly.
Another strip has Calvin trying and failing to persuade Susie to stand on an "X" mark in the snow, foiling his plan to spring a pile of snowballs on her.
Susie also gets Calvin passing notes in class. She tells him to pass on the note, but not to read it first. The note, naturally, says: "Calvin, you stinkhead: I told you not to read this. Susie."
A more successful version uses a series of "Important message, this way" signs, culminating in "Important message : Look out!" and a gigantic snowball dropped from above.
In one strip, Dilbert tells the Pointy-Haired Boss not to touch the "prototype", or he'll be shocked. Naturally, the boss's thought bubble reads "Must touch", and he touches the thing, and gets zapped with a ridiculously huge surge of electricity. In the third panel, Dilbert tells the boss not to touch it a second time, and the boss, who is severely charred by the first shock, thinks "Must... touch... second... time."
Wally makes use of this trait in later comic, when Asok accidentally erased the entire customer database. He gives the boss Asok's laptop, and says that this is an unstable prototype and should not be touched. When the boss immediately does, Wally blames the boss for just now erasing the database.
In another strip, Wally - again - tells Dilbert, with the PHB listening, that studies show you can identify a natural leader by the way he says the word "gullible". This causes the PHB to start yelling "GULLIBLE!" repeatedly at the top of his lungs, and Wally to say, "Sometimes I love my job."
In one strip of the newspaper comic Frazz, we find out that the eponymous janitor has installed a box designed to make the kids ask, "What's that for?" so that he can tell them to leave it alone. The box actually doesn't do anything, except to distract them from the fire alarm.
ThisGreenside cartoon. A marine stands in the middle of a room full of signs in bright colours with enormous lettering telling him not to press the button. He is pressing a large red button. Two scientists just outside the room are looking in through a window and the first is telling the second "I told you so."
In a Prickly CitySunday Strip, Winslow sees a button with a "Do not push" sign. After some careful idleness, he pushes it in the penultimate panel. In the ultimate one, he complains, "If you break the rules you deserve a boom."
The Far Side had a cat following signs saying "cat fud" leading into a washing machine, behind which a dog was lurking in breathless anticipation.
In one Garfield strip, Garfield finds a fence with a gate that says "Do Not Open This Gate!" At first he's eager to open it ("That's an open invitation if I ever saw one!") then he thinks maybe it's a bad idea ("On the other hand, it might not be wise to open this gate.") Then he thinks that never stopped him before, and decided to take a peek... Only for a mob of dogs to rush out, trampling him. In the last panel, Garfield is flat on his back, and groans, "Curiosity mangled the cat..."