Hoist by His Own Petard/Web Comics

"Othar's twitter: When Othar Tryggvassen builds a deathtrap he does a pretty damn good job of it. Some would call this irony. Everyone else, stupidity. Sigh."
 * Subverted in Narbonic, where every self-respecting mad scientist expect their creations to at least attempt to destroy them. Helen repeatedly points this out to Artie: "Let's face fact. As you are, you'd go on a pretty poor rampage.".
 * Played straight when the subject Helen has been experimenting on for the entire run of the comic
 * The Fat Guard who tortured poor Fluffles in Goblins.
 * The demoness who used an Exact Words Deal with the Devil to claim ownership of Dies' Horribly's soul falls foul of the terms of her own bargain.
 * Referenced in Questionable Content #628.
 * Girl Genius: "He threw a bomb at me!"

"Stinky: Scobee, this rivalry with Borzoi has got to stop. We're running out of spare parts!"
 * In Everyday Heroes, while J.P. Wunsch is in prison, he tries to rebuild his power cuff that turns him into Wrecking Paul.
 * In Eight Bit Theater, Thief gains his class change to Ninja by stealing it from the future. Much later, after the other Light Warriors get their classes taken away, Thief avoids this. Then his past self shows up...
 * A somewhat meta example in xkcd. (Check the Alt Text.)
 * It's also a Call Back to this comic.
 * In Cuanta Vida, the BLU Medic is killed when he's (accidentally) injected with the injection he meant to use on Blue Scout/Jeremy.
 * In The Order of the Stick, Xykon sends several goblins to their deaths simply to observe the effects of walking into Dorukon's Gate. They invariably explode, but Xykon is a For the Evulz kind of guy, and thinks it's funny. During the fight with the PCs he shatters Roy's ancestral sword, and Roy picks him up in a bout of Unstoppable Rage and throws him into the gate himself. Of course, it takes more than the destruction of his body to permanently destroy a lich, but it does set his plans back months while he seeks out a new Gate.
 * Tsukiko is a necromancer with a rather disturbing fondness for her undead creations, believing that, since the undead are the antithesis of life and Humans Are Bastards, the undead must be misunderstood, lovable souls. Redcloak demonstrates his own belief that the undead are nothing more than tools to suit a purpose by using a 'Command Undead' spell to force Tsukiko's thralls to devour her and each other.
 * Girly: Called out by Clone Sidekick Hammergirl, as she is smashed to death with her falling hammer to reveal a hollow center!
 * In Juathuur, the juathuur deserters with whom Meidar is fighting left the path in large part because the Secret Test of Character she had for each of them left them disillusioned. One particular former juathuur, Rowasu, kills her in the end.
 * Nature of Natures Art: SV, why did you decide that harnessing the power of malice would be a good idea? Sure, it's a rather unorthodox definition of malice, but that doesn't change the fact that it ultimately killed you - or "[devoured] you as you [devoured] it", as you put it. At least you realised it at all.
 * Spacetrawler:
 * Yuri goes full cyborg, incorporating more and more weapons into her body. Martina decides that Yuri is going too far and needs to be restrained. To incapacitate Yuri, she uses a bomb that Yuri herself had commissioned from the Eebs.
 * created the brain clamps that would eventually be used to cripple their psychic powers and rob them of free will. In fact, they were the ones who started brain-clamping each other in the first place; when outsiders decided to enslave them, all that was necessary was to kill the half-dozen of them who remained unclamped.
 * Homestuck:
 * Equius's bodybuilding obsession leaves him strong enough that a rifle falls apart in his hands ... a few strips later, Gamzee wounds him with a bow and arrow, then walks slowly across the room to garrote him.
 * Vriska  after he tries to kill her, killing him.
 * As the Thief of Light, Vriska has apparently complete ownership over fortune, with the ability to alter luck-based events to her whim. When she refuses to back down from going off to fight Jack, Terezi makes her a bet her ego could not refuse - a simple coin flip. Heads, she stays. Scratched heads, she goes. The double entendre of the word was intentional and understood by both to be death. Vriska forces the coin to yield the scratched head, gambling that Terezi did not have the courage to go through with it. She was wrong in the, and dies in accordance with their agreement.
 * In Mitadake Saga, Kira . This is realized far too late.
 * Twenty First Century Fox has a few incidences that have this trope:
 * Subverted here: Col. Tora Scobee, a bionic wolf, plants a bomb in the office of his rival, Borzoi, who is a Russian Wolfhound. Borzoi sees the bomb just as Scobee is getting into his car, and Borzoi chucks the bomb at the exact moment Scobee is inside. The last scene with Scobee in a full-body cast and his boss, Stinky, berating him.

"Petey: You must demonstrate the ability to defend the surface of this planet. A Frigate armed with light shields and a fully operational plasma lance has just entered orbit. All you need to do is shoot it down. Petey: In ten minutes it will be overhead, at which point it will destroy this building and any who remain inside."
 * Played straight when two tiger-girl Mooks learn too late about the direct way for sinking a ship.
 * Petey in Schlock Mercenary just loves to do this sort of poetic justice, to the point of turning it into a standard procedure.
 * A government explicitly responsible for defenses cuts some expenses on this part and he had to intervene?

"Petey: but I expect that you will get quite a rise out of them when you explain to them that these proceedings will be made public as part of the extradition process."
 * U.N.S. requests the extradition of mercenaries who blew up a TV company, pulled an insider job in process and planted fake evidence. After the local government itself . The case is clear, so he's most eager to cooperate...

"Detective Fitzsimmons: These thugs might get away with blowing up buildings, but there is still the question of illegal soldier boosts. Petey: Question? There is no question at all. The human Toughs have illegal boosts, which they acquired via a fugitive U.N.S. researcher. Petey: Oh, wait... you mean the "does the U.N.S. want the details of that technology released as part of these proceedings" question."
 * ...and since they didn't learn fast enough to drop it fully:

"Lucifer, The Light-Bringer: Actually, before I was evil? I was an archangel. (Cue Curb Stomp Battle)"
 * Someone purchased large parties of weapons from Planet Mercenary, but suddenly cancelled orders right after a civil war that was about to start all over the human space didn't. Immediately after receiving report about these unexpected expenses, the boss of Planet Mercenary covered them simply by selling these clients to U.N.S. intelligence, very interested to know who was behind this unholy mess. Not dealing in good faith works both ways.
 * Campaign Comic One Piece: Grand Line 3 Point 5 sees Cory fall into this several times, due to his taking several Flaws he figured would never come up. For instance, he gave Zoro the Loyalty flaw, thinking he was gaming the system since the Chaotic Neutral swordsman never answered to anyone. Then he agrees to join Luffy's crew.
 * In the Attack of the Clones portion of Darths and Droids, Pete is forced to run R2-D2 through the death trap maze he created when he was GM as punishment for putting the others through it.
 * In Sinfest, Lil' Evil uses his slingshot to attack God. He fails to consider gravity.
 * In Bob and George, in the tourney, Mouseman thinks he can obviously defeat Catman because the cat always gets caught in its own trap. His attempt to lure Catman into it means he gets caught.
 * Sluggy Freelance in the Haunted House arc. The bride refused to kiss the groom until he will "set things right" and ran off. To never be seen again. It turned out that her demand was to murder his previous mistress. This kind of worked... and kind of backfired.
 * ...and then the next tenant, Brad Trivol who "was not the nicest man. He was always looking for a fight, never backed down from one, and never lost one. That is, until taking his own life a week after moving in." - that is, these two now semitransparent clowns forced him to reenact Henry's suicide. This didn't work well for them in the end. His last words before shooting himself in the head were "Never be rid of you? I'm coming for you!"
 * In Casey and Andy, the demon Azrael is attempting to unseat his master Satan by draining away all of her evil. After having drained most of it, leaving her an almost-powerless shell, Satan manages to force her way to Azrael's side and basically dares him to finish the job. Smugly he does, taking her "last speck of evil" in full confidence that it will render her nonexistent. Instead...