Just completed Storm in a Teacup: The Physics of Everyday Life by Helen Czerski. I learned that yes, it really IS true that bread will land on the floor butter side down. Nothing to do with butter. Laws of physics, not just bad luck.
No, the weight of the butter is small compared to weight of bread. As the link shows, angular momentum. When the bread starts to slide to edge of table, there is a microsecond when the gravity pushing the leading edge (that is hanging off table) down is exactly balanced by electrostatic forces keeping bread on table. Creating a seesaw around the center of gravity. If motion stops there the bread could stay put indefinitely but it is continuing to move. In a few more microseconds gravity exerts a greater force and the "seesaw" is now rotating, angular momentum flipping the bread over. It lands butter down not due to anything in the butter but simply because butter had been on top. When bread flips, butter now on bottom. As link shows, when bread is simply dropped, not knocked to edge, it also spins due to angular momentum.
