Jack Up with Phlebotinum

Bob is someone with certain... lusts in life. It may be something as simple as loving to eat a lot, or something as self-destructive as heroin. The point is, he's jonesing, and he's jonesing bad. And suddenly, he finds a stash of what appears to be the good stuff! Jackpot!! Bob then consumes or shoots up the stuff...

...Only to suddenly mutate into a hideous monster, melt into a pile of goo or keel over dead!!

Poor Bob, he just had to Jack Up With Phlebotinum. Instead of the good stuff, the stuff he consumed was actually some sort of harmful substance or Applied Phlebotinum, and he has now become either a casualty of some bizarre effect or something less than human. Either way, Bob himself has now become an integral part of the plot somehow. Perhaps his demise sets events in motion that drive the plot, or perhaps he becomes some sort of monster and is now the antagonist. Bottom line is he really shouldn't have done the stuff.

This trope involves the poor fellow imbibing the nasty stuff by accident. If it was deliberately caused by someone else, it's Tampering With Food and Drink or Slipping a Mickey. It also differs because those usually involve more mundane substances, while this trope involves taking some VERY bad shit.

Fantastical or exaggerated subtrope of Drugs Are Bad. Compare Mushroom Samba, where the character accidentally imbibes something that blows his brain instead of his body.

Comic Books

 * In The Sandman, a former girl of John Constantine's swiped an artifact he had thinking it was drugs. It turns out to be Morpheus's Bag of Sand. This is a very bad thing, as it basically turns her life into a literal living nightmare.
 * In Lucifer, demonic succubus Lady Lys gets a hold of Christopher Rudd, one of Hell's damned, as her personal plaything. Rudd, at first relieved he's being spared the suffering he's endured for centuries, comes to resent the situation, since he believes himself damned worse for consorting with demons than for paying penance. Lys, like many of Hell's denizens, is addicted to Pain(capital P), which is the distilled essence of the souls suffering in Hell. Rudd, however, distills ACTUAL Pain, pain as humans feel it, and gives it to Lys. When Lys imbibes it, she feels the actual pain of the souls she's tortured and played with, making her feel remorse and shame for her past actions. The experience turns her from a hedonistic slut into a broken, emotional wreck.
 * One of the origins of the Sentry of Marvel Comics (He has a Multiple Choice Past) is that he was a junkie who stole the Super Serum in hopes it would get him high. It turned out remarkably well for him...except for the side effects.
 * Like the Sentry, but far less powerful, minor Spider-Man villain Freak started as a drug addict who consumed a mutagen thinking it was a new designer narcotic.
 * There was running plotline in Brian Michael Bendis's Marvel stories about regular people taking MGH, a drug that grants super-powers along with a high.
 * In Grant Morrison's X Men run, a new power-enhancing stimulant called "Kick" starts taking the mutant community by storm. Turns out it's
 * A Spider-Man story had the Absorbing Man transforming himself into cocaine, then being cut up and sold to unsuspecting addicts who got their own superpowers.
 * Wolverine once fought an Eldritch Abomination called Sporr, which had been blasted to molecules millennia before by the Celestials; it reemerged when someone deliberately overdosed on cocaine that was grown on the mountain where it "died" because said cocaine gave its users limited super-powers.
 * Legion of Super-Heroes mainstay Bouncing Boy got his powers by mistaking an elasticity drug for a bottle of soda.
 * The Suicide Squad once thwarted a scheme by voodoo-using New Orleans gangsters to distribute free heroin samples to addicts that would turn them into a literal zombie army.

Film

 * In Pulp Fiction, Mia Wallace finds what she thinks is cocaine is Vincent Vega's coat jacket, and snorts it up. It's in fact VERY pure heroin, and she ODs on the spot.
 * In the 1980s horror film Leviathan, the crew of the underwater mining facility find a sunken Russian ship and inside find what they think is vodka, which they gladly drink. It's actually a solution which has The Virus, mutating the crew into a hideous monstrosity.
 * Return Of The Living Dead 5: Rave from the Grave has kids in raves gulping down pills of what they think is a rave drug like Ecstasy, but it is actually Trioxin, the chemical phlebotinum from the movies that turns people into zombies.
 * In Beyond Re-Animator, a junkie prisoner manages to get some syringes of Re-Agent, which he promptly shoots up. Hilarity Ensues.

Literature

 * The Stephen King short story "The Ten O'Clock People" involves a relatively mild example: a certain level of nictotine and withdrawal symptoms found in smokers trying to quit-- and only in smokers trying to quit -- gives them the power to perceive.
 * In another Stephen King story, "Gray Matter", an alcoholic slob drank a can of skunky beer and started turning into a flesh-eating fungus-like creature.

Live Action TV

 * On Stargate Atlantis, Ford is injected with the enzyme Wraith use to strengthen humans so they can survive the feeding process, but doesn't get eaten afterwards. As a result, he gains super speed, strength, and stamina. The enzyme proves highly addictive, severely impairs judgment, and the withdrawal process is not pleasant. Ford ends up going rogue and gathering together a band of guys who use the enzyme to aid them in their fight against the Wraith.

Western Animation

 * South Park: Cartman ingests Kenny's ashes, thinking it's chocolate milk mix. Now he has Kenny's soul trapped in his body.
 * There's this episode of Code Lyoko called Temptation. Jeremie translates a part of Franz Hopper's diary, and uses the info to invent this weird headband. It's supposed to raise his intelligence every time he uses a Return to the Past (go with it). He first notices the side effects, like irritability and fainting spells. Then he realizes that all the calculations he made while using the headband were wrong, meaning it had no positive effects whatsoever (except to make him a temporary Memetic Badass).
 * On The Tick, Dinosaur Neil, a paleontologist in a dinosaur costume, accidentally eats dinosaur genetic material thinking it was pasta salad, which turns him into a real dinosaur.

Real Life

 * Cats perceive antifreeze as sweet and tasty; it quickly kills them via liver damage in very low doses.
 * In the early 20th century, radium was sold as a wonder drug because of its healthy glow. A business tycoon named Eben Byers who started obsessively chugging a radium serum eventually perished, leading to the squicky Wall Street Journal headline, "The Radium Water Worked Fine until His Jaw Came Off."
 * Often happens with methanol. This substance looks like ethanol, smells like ethanol, tastes like ethanol and even produces buzz like ethanol. But if you're lucky, it'll burn your eyes from inside for good, and if you're unlucky, it'll kill you.
 * Russians have devised a folk way to tell methanol from ethanol. You make a small spiral out of copper wire, heat it with a cigarette lighter and drench in the suspicious alcohol. If you smell "morgue" (formaldehyde gas), this is deadly stuff.