Properly baked dwarf bread is always made by a lethal chef. Used less as a food and more as a weapon, its main ingredient is apparently gravel. Its most useful purpose when used as rations is to make everything else look edible. Note that this is intentional on their part, and they can cook other dishes that are perfectly edible to any race (at least, if you don't mind rat).
Subverted by the Vimes household; Lady Sybil is a bad cook, but Sam has spent so much time eating low-quality food on the streets of Ankh-Morpork that he actually enjoys it. As long as he can pick out the lettuce. Or any of the vegetables. And the fruit, too, for that matter. His favorite foodgroup is Burnt Crunchy Bits...
Albert counts too, but it's not so important when you're the cook in Death's household. He firmly believes in grease, fat and black gritty bits, and is the inventor and sole eater of fried porridge. It eats spoons.
CMOTDibbler sells sausages that qualify as food on a good day
The numerous Dibbler copies also have their own barely edible wares: Disembowel-Meself-Honourably Dibhala's funny-colored antique eggs, Al-Jiblah's highly suspicious cous-cous, the terrible yak-butter tea made by May-I-Never-Achieve Enlightenment Dibhlang and the unmentionable blubber of May-I-Be-Kicked-Into-My-Own-Ice-Hole Dibooki (main ingredient: exploded whale), the green beer of Swallow-Me-Own-Blowdart Dlang-Dlang and the meat pie floater in pea soup with tomato sauce (regional speciality of the last continent).
Bungling Inventor Bergholt Stutly "Bloody Stupid" Johnson took up cooking at a few points in his life. His attempt to make a pie took out a significant portion of Ankh-Morpork. The actual recipe was fine; it was just that Johnson approached it with his usual indifference to measurements, resulting in something so large that it achieved culinary instability and exploded under its own weight.
Mrs. Samuel Whiskers from Beatrix Potter's Tale of Samuel Whiskers and the Roly Poly Pudding.
The outdoor humorist Patrick McManus has written extensively about growing up in a household headed by his hard, fearless, super-competent mother, commenting that the only thing she couldn't bend to her will was food. He learned to "scrape off the burnt parts". He also has written on occasion about the dangers of eating camp cooking. Don't ever eat the Green Hash.
There is an entire rhyming children's book about this very subject called The Great School Lunch Rebellion.
Hagrid's poor attempts at cooking are the butt of many jokes in Harry Potter.
Inverted in a Roger Zelazny book, Isle of the Dead. The main character employs an alien chef who can prepare the finest specialties of human cuisine, but owing to biology and personal taste, considers the dishes to be something between vomit-inducing garbage and toxic chemistry experiments.
In the Chalet School series, some of the girls' cooking failures are played for laughs in some of the cookery classes. In The Chalet School and the Lintons, for instance, Cornelia Flower uses garlic cloves as flavouring for apple pies rather than normal cloves, and in Carola Storms the Chalet School, Carola Johnston's class fry their jam doughnuts in cod liver oil.
Jay Leno's children's book If Roast Beef Could Fly has Jay's father, who attempts to do a BBQ every year but fails miserably in someway - he even says that his father throws the roast when it's done.
At least one of Mercedes Lackey's SERRAted Edge books features a Sidhe warrior attempting to cook breakfast. Mind, the Sidhe ordinarily magic their food out of thin air... The sequence includes such gems as breaking the number of eggs required by the recipe and then "carefully picking out most of the shells" and figuring that hey, tomato paste, Tabasco... Both red sauces, a 1-1 substitution should be perfectly cromulent. This disaster actually becomes a major plot point instead of just a random funny. He gives up after recognizing his failure and conjures the breakfast - but doesn't do it sneakily enough, blowing his human cover identity.
The searat captain Slipp in The Bellmaker claims to be a cook rather than a pirate when trying to get into the Abbey. Of course he's told to prove it. His patented "skilly an' duff" contains a wide range of vegetation of dubious non-toxicity and the smell is compared to that of a compost heap, thus blowing his cover.
Molly Carpenter from The Dresden Files is a borderline lethal chef. According to Harry she one time burned a boiled egg, and refers to her kitchen attempts as "committing dinner." At least she can make coffee.
From Codex Alera:the Vord Queen's attempts at cooking tend to fail. Then again, so do all her attempts at acting human.
While we never actually got to read about Shae's cooking in A Song of Ice and Fire , she did mention that "every man who tasted her cooking told [her] what a good whore [she is]."
In Like Water for Chocolate by Laura Esquivel, the main character is actually a Supreme Chef most of the time, but she has the semi-magical ability to channel her emotions into her food. When baking a wedding cake for her sister's marriage to the man she wanted to marry herself, her despair affects the cake and makes people sick to their stomachs. When tossing leftovers to the chickens she raised after having an argument with her sister, her anger makes the chickens attack and kill each other after eating what she feeds them.
In the children's book Olson's Meat Pies, a once-gourmet restaurant runs out of ingredients and in desperation starts putting all sorts of inedible objects in its pies. Popular outcry follows.
In Coraline, the titular character's father is somewhere between this trope and Cordon Bleugh Chef in that most of his meals are lots of different flavors implemented badly — of particular note is the unevenly-cooked homemade pizza which had pineapples on it served at the end.
Moiraine and Siuan are both revealed to be lethal chefs in New Spring, when custom requires them to bake an Aes Sedai a pie shortly after their initiation ceremony. Moiraine's is inedible (and justified, as she's a noblewoman who never needed to learn how to cook). Siuan's is more subtle; the Aes Sedai eats it all, proclaims it very good—and then has to run for the privy as well as get magical healing after.
Miss Mush and Mr. Pepperadder from the Wayside School series. Subverted in that they aren't so much poor cooks as poor quantity cooks: when they only have to cook for two or three people, it's gourmet quality. However, Wayside School has 4,000 people in the building. Her most popular dish with the students is nothing. The kids really dread the days she runs out of it, and they have to get something else.
Alfons Zitterbacke from the children's book from East Germany with the same name. Once in summer camp, Alfons and his buddy Bruno volunteer for cooking. Which they've never done before. They try Makkaroni with tomatoes. At first they don't use water, and half of the noodles get burnt. In the second try, they use more water, then throw the tomatoes in, but notice that the result looks more like soup and tastes "funny". They decide it lacks salt, and add three pounds of it. No wonder one girl thinks she's poisoned when trying to eat it.
In Our Dumb World, paella is described as containing a long list of things both edible and not, concluding with "...and anything else in arm's reach that's not too heavy."
The Justicar from Paul Kidd's trilogy White Plume Mountain, Descent Into The Depths Of The Earth, and Queen Of The Demonweb Pits is a pretty good camp cook, but that doesn't stop Escalla from once remarking that "real meals don't look up at you from the plate and offer to negotiate". Escalla herself has a tendency to over-sweeten food.
Justicar: Escalla, do we even have the ingredients for any of the food in your recipe book?
Escalla: We can substitute with fresh foods from the wild! We'll do beef ragout.
Escalla: Beef, tree-frog, same difference. It'll be a blast!
There's an old anecdote about a maid who's supposed to bake a goose. Which she does by putting it right into the oven, as it is. With feathers still on and innards still in. As the narrator tells us, it was a big stunk.
James McDonough, author of Platoon Leader, also wrote a tactics manual. Its framing device was that the protagonist was in purgatory, having to repeatedly fight battles to get into heaven. How did he die? He ate three MREs in rapid succession.
In Penny from Heaven, the eponymous character's maternal grandmother, Me-me, is one. Her liver is described as worse than her pot roast, which is worse than her beef stroganoff, "and you don't even want to know about her meat loaf". On the flip side, her Italian extended family on her father's side is a very long line of Supreme Chefs.
In the Sector General book The Galactic Gourmet, a sous chef changes what is though to be an erroneous recipe (reasoning that the chef couldn't possibly have purchased decades worth of nutmeg all at once), resulting in mass nutmeg poisoning.