When I was a kid, my grandmother and I tried to make a chocolate mousse. It was a disaster from start to finish.
I don't remember a whole lot, but I do remember her starting by saying, "First we need [some number] cups of milk," and promptly pouring the milk into her sugar bowl. Then, replacing the sugar, she managed to lose hold of the bag and poured sugar all over the kitchen table.
The final result was anything but mousselike - it was soup in the refrigerator, and harder than granite if we put it in the freezer. The strange thing is that I'm pretty sure we were following the recipe fairly closely.