Let's see... the first time I worked with chocolate, I tried to melt it in a double boiler and I think it seized from getting steam into it or something. At any rate, it was lumpy and I assumed it just wasn't melting, so I then proceeded to heat it for so long it burned. Yes, I seized AND burned the same 8 ounces of chocolate. Finally we just said "well, it's not melting, let's just put it in the recipe anyway," and it came out pretty well aside from tasting like burned chocolate. :-/
The first and only time I tried to make beef stock, I still haven't got a clue what went wrong. I got really good quality cuts of oxtail and shank, slow-simmered it with onion, celery, carrots and herbs for many hours. It was a really rich color, had an incredible amount of gelatin (like tire rubber when it chilled in the fridge!) and tasted pretty much like dirt with no meatiness at all. :-(
Tried to make gluten-free brownies from scratch after my SIL was diagnosed with celiac. They came out a totally bizarre texture, which I have never seen food achieve before or since. Threw them away and brought a few pints of ice cream to the dinner party instead. (I later learned how to make pots-de-creme, creme brulee, and flourless chocolate souffle for GF desserts, to rave reviews...)
I've had several mixtures break completely inexplicably - a ganache, a custard, etc. Tried and true recipes, no idea what happened. The ganache broke when I combined chopped chocolate and heated cream and stirred. The custard broke when I added a vanilla bean. WTF?
When I was about 11 or 12 years old I was invited over to another girl's house (who I honestly didn't like very much but you know it is when you're a kid... Mom decides you should be friends with her friend's kid or whatever, and you're stuck). We decided to make chocolate chip cookies but I'd never really done any baking without an adult and didn't realize how recipes worked. We dumped in all the ingredients at once. Before we could attempt to mix it together her mom came in and got all mad at us for doing it wrong and wasting all those ingredients.