When I was at school we made a quiche as part of our Home Economics class, I was 15 years old. A couple of weeks after the Home Ec class, I decided to give my Mum a rest from cooking and volunteered to make the quiche again; as it turned out ok the first time my Mum agreed.
During the quiche making, my friend telephoned so I chatted with her for about 30 minutes and took a break. I went back to the cooking and all went well, so I thought. I proudly served the quiche up to my parents and they both took a mouthful and as they were eating it they were looking at each other weirdly. I was then informed I had forgotten to put the cheese in, so it was effectively a ham and egg tart and it just didn't taste right without the cheese! I never attempted to make a quiche again after that!
My other mistake was attempting a chicken tikka masala after I had been taught it as part of a cookery course I went on in Kerala, India a couple of years ago. I followed the instructions but it turned out so hot we were crying as we ate it...it was almost inedible but we have a high threshold for spiciness, so it was sort of ok. I think this mistake was a case of the chillis being a different heat between countries, rather than anything I did wrong.