First thing that comes to mind was the first time I tried making brownies when I was about 12 years old. My friend and I bought all the stuff and followed a recipe, and we thought we were geniuses for deciding to add a little peppermint extract to make them into 'mint chocolate' brownies. So we baked them up, they looked beautiful. We cut them up with anticipation and we had our first taste.
It tasted like minty DIRT. They were bitter and horrible. It took us awhile to figure out what went wrong, then we realized we were supposed to use MILK baking chocolate and we bought UNSWEETENED baking chocolate by accident. Didn't even notice. It was so nasty and the taste stayed in our mouths for hours.
First Thanksgiving I spent away from home, I decided I wasn't going to cook anything or make a fuss. Then, at the last minute, I changed my mind. I went to the grocery store but they didn't have any decent turkey breasts left so I bought some ground turkey instead. I never worked with the stuff before, but I figured I'd just 'throw' something together.
I got home, plonked the stuff in a bowl, added chopped onion, some sage & thyme, some egg and bread crumbs. I made this 'loaf' and shoved it in the oven. I made some sweet potatoes and green beans to go with it. I got it out, let it cool, and sliced into it.
Holy crap it was so disgusting. It was like trying to eat slices of stiff, dry kitchen sponge with raw onions in it (they didn't soften at all). So I wound up tossing it all out and having sweet potatoes, green beans, and hummus on pita bread that Thanksgiving, haha.