I love this thread! My mom worked a lot so was not much of a cook so I only decided that I really needed to learn how to cook about 7 years ago. I know I have several, but I can only think of 2 recent ones.
I had my DHs family over for a BBQ and decided after dinner to whip up some brownies. Well I was a bit distracted while I was mixing them up, but I finished mixing and put them in the oven. About 15 minutes later, I was like, why are these eggs sitting on the counter...OOOPS.. those were supposed to go in the brownie batter!!! LOL So, out of curiosity, I just let them finish cooking to see what the results would be like. Basically it was like "brownie crumble", it didn't taste bad, but the texture was dry and crumbly. I ended up saving it and using it as an ice cream topping, which wasn't too bad.
The other one was this weekend, I had never made german potato salad before, and it sounded good, so I looked up an online recipe. I doubled the recipe, and added the vinegar, sugar, etc., but it seemed like a lot of sugar, so I only put in the original amount of sugar. I should have listened to my gut, for some reason even with just the original amount, it was waaaay too much sugar, and it completely overpowered the vinegar. I tried adding more vinegar, but you could still mostly taste the sugar. They weren't horrible, but they weren't the flavor I was hoping for.
Oh and I will add one from my childhood. My mom made swiss steak, which was one of my favorites, so I took a great big bite expecting my favorite dish, instead I got a mouth full of very hot sauce. I about choked and I think I even spit it out saying "mom, this is really hot" and she told me I was being a picky eater or something like that, and then my dad took a bite and said, "ummmm, she is right!" It turns out that my mom was shopping at the local mexican grocery store and bought cans of, what she thought was tomato sauce, but was really hot sauce. My mom: "Well it had pictures of tomatoes on the can". Needless to say, we did not eat it, I think it was even too hot for the dog.
Hopefully I can think of some more later. :)