If any human being becomes too self-aware of their brain's functions, such as the reflex arc or the chemical reactions in their brain, their brain deterioates, scrambling every nerve ending in the body before shutting down. This is the worst pain known to man; the triple placebo pneumatic bypass.
Some say this is what the matrix does when it looks like one of its captors are going to break away.

This can also occur when a human being spots a glitch in the Matrix, e.g- George Bush Junior being re-elected.

When you eat dirt for the first time and leave your family to build a shack in the woods