Sounds like the inverse about the physicist joke (I'm remembering this incorrectly, I'm sure)

Farmer:  How are you going to increase my milk production?

Physicist:  First, assume a spherical cow…

Assumptions which are in contradiction to fact (I don't allow for the Earth's shape being roughly spherical in my day-to-day life) or theory (nor do I tend to worry about relativistic or quantum mechanical effects on my ride to work) are made all the time, even by people who do know better.

From what I remember of my high-school French, it's chock full of idioms, and these idioms can easily confuse the unwary, a category my French language skills never left.