I believe the worst [philosophical] mistake of all is the cluster of views known as Dualism, Materialism, Monism, Functionalism, Behaviorism, Idealism, the Identity Theory, etc. The idea these theories all have in common is that there is some special problem about the relation of the mind to the body, consciousness to the brain, and in their fixation on the illusion that there is a problem, philosophers have fastened onto different solutions to the problem
This mistake goes back to the Ancients, but it has received its most famous exposition by Descartes in the seventeenth century, and has continued right through to the present mistakes such as the contemporary Computational Theory of Mind, Functionalism, Property Dualism, Behaviorism, etc.
A mistake of nearly as great a magnitude overwhelmed our tradition in the seventeenth century and after, and it is the mistake of supposing that we never directly perceive objects and states of affairs in the world, but directly perceive only our subjective experiences.