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Yeon from Tower of God has only cooked for the team once. When she wanted to try again for Viole's sake, the rest of the team tried their best to dissuade her, a dark background of doom appeared, and Miseng even started to cry.
All Over The House regularly feature's the unfortunate results of Tesrin's attempts to cook.
College Roomies From Hell Marsha is a webcomic lethal chef, causing stomachs to be pumped and a single bite from one of her cakes sending someone to the hospital. Mike doesn't dare say anything, suspecting (correctly) that she'd dump him for it; her roommates openly complain and go to great lengths to keep her away from the kitchen, but she seems to think this is just spite. This is partly because her father is a master chef; she insists that being a good cook is in her blood.
Brüno had the title character staying in an isolated house with the writer Stanley, who managed to ruin everything he cooked for her. Subverted at the end of the storyline, where he reveals that he's an excellent cook; he initially made inedible food because he wanted to be left alone, and kept doing it because Bruno's reaction was so hilarious.
Pat from Achewood may not be a completely lethal chef, but the vegan meals he serves to the other characters aren't all that tasty, either. In one strip, he offers them some of his homemade chocolate-covered cherries — which turn out to be stewed prunes covered in carob. For this injustice, Todd shanks him in the leg with a toothpick.
Crystal from Sluggy Freelance is apparently a lethal bartender. There's her infamous "Survivor Nights" (in which her patrons vote off her various alcoholic concoctions from the menu), and the fact she's created drinks such as Prince Charles (tastes like ear) and Cheeseburger Margarita (exactly what it sounds like).
Ace from Too Much Information is in all other ways The Ace, but he likes hot food. Really hot food. As in, puts hot sauce on his pancakes. His housemates won't let him share in the cooking duties. Anything he fixes is bright red from all the hot sauce, and is edible only to him and some kittens with black holes for digestive systems.
Ma-ri and her entire family in Orange Marmalade are this with any food outside of pork, justified because they're vampires who can't eat anything besides pork.
The Golden Trough is an all-you-can-eat restaurant staffed by lethal chefs who cook with low-quality ingredients. Parents take their children there as punishment.
The Mayor's mom was a demolitions engineer, not a baker. Her attempt at pancakes is never shown, but it gave the mayor shrapnel wounds.
Several characters in Girl Genius seem to fit this trope.
Moloch's cooking is so bad that a fellow inmate claims that she would rather eat his engines — he only has the job because the job goes to the newest guy since no one wants to stay in the evil sentient kitchen built for a literal Lethal Chef.
Theo mixes Gilgamesh a drink that actually causes him to stop breathing for a time (though this might have more to do with Theo preferring his liquor to be 200 proof or higher, based on dialogue).
Gil himself apparently has made a drink out of toothpaste and hedgehogs (that's actually quite good).
Oddly enough, Agatha seems to be a lethally good cook — at one point she makes a cup of coffee so "perfect" that more or less put the drinkers into stoned euphoria. Also referenced when Agatha first enters Castle Heterodyne, where all of the Empire's worst criminals are sent. When asked what she's in for, she replies "I poisoned 37 people who complained about my cooking" in order to avoid kitchen duty (it doesn't work).
Zola, if we'll believe Gil:
Agatha: She uses poison?! Gil: Well, I always thought she just couldn't make coffee. But now I'm not so sure.
In the older archives for the webcomic Absurd Notions the horror of the dining hall is only hinted at, but the author's notes go into great detail. Suffice it to say that their spaghetti sauce was legally actionable.
One three-part story featured an "Iron Chef Ramen Noodle" cook-off between Igor and Carson the muskrat. Carson's entry consisted of flavouring and generic tofu dogs, and prompted the question "This used to be organic?" from the judges. He wins anyway, despite Igor's entry being "like tasting clouds", because his entry reminds the judges of college dorm food - "The best years of our lives!"
Earlier Igor purposefully invoked this trope with his Igor Bars
Early in Exploitation Now, Bimbo made some clam chowder that was not only inedible but, when flushed down the toilet, summoned an Eldritch Abomination that destroyed her apartment building.
Two appear in 8-Bit Theater. There was the restaurant with biscuits made out of "solid goddamned lead," and then came Red Mage's shoe sandwiches:
Red Mage:And they didn't kill us! You guys never thanked me for that.
In GingerDead and Friends, Ennui/Lenoir is a Lethal Chef less because she's bad at cooking (she doesn't appear to be), and more because she forgets that living people don't usually like poison in their food (she is quite probably dead, and the poisons don't affect her anyway).
Nodwick: Piffany is an inversion, and is so good at baking that cosmic war can be averted by giving the gods her brownie recipe. Yeagar, on the other hand...
Artax: Don't touch that! It's a deadly mould! Yeagar: How do you know that? Artax: It looks like your cooking!
Our Little Adventure has Pauline. One such instance involved a dead bird Angelika was going to throw in Julie's tent as a prank, and the other involved 'things that are like Chicken' and 'things that are like vegetables.'
Evil Plan The Webcomic: Problem: Little sister Kristen's food kills. Solution: Elder, mad scientist brother Stanley uses it to kill his foes. By filling a vat with it and dropping them in.
In MSF High there are at least two, Alouette and Lana. While their food may technically be edible, it tends to become alive and hunger for flesh.
In Drowtales, Chirinide attempts to cook a bird for her faithful bodyguard Shan'naal. She fails to realize that you have to pluck and gut the bird before you put it in the oven. She also tries to start said (non-magical) oven with an enormous fireball. Tur'geis says he'd eat it anyway, and Ariel actually remarks it smells good. This leads Nuru'lara to conclude that Sarghress-born have no taste.
For Slice of Life, since it is set in a bakery, everypony falls prey to this trope sooner or later.