I once forgot to put the sugar in some pumpkin pies I was making. As I put them in the oven, my thumb slipped into the filling a bit. After I closed the door, I licked my fingers and discovered my mistake. I had to dump them in a bowl, add sugar, then refill the pies. They were messy, but still tasted good.
I often overcook salmon and it gets rubbery.
I made a batch of orange/ginger marmalade last year that didn't set, so I use over salmon. It is delicious unless I overcook the salmon.
While on a low carb diet, I once made some artichoke cheese squares that were absolutely horrid. Dry and tasteless. I tossed them.
Though I am sure I made some mistakes as a beginner cook, such as getting baking powder and baking soda mixed up, I can't remember the details, although I am sure a few batches of cookies and a cake or two got tossed out.
Nowadays, my kitchen mistakes usually involve burning things due to multitasking. CC and broiling meats are not activities that should be done at the same time.