  An analysis of errors in measurement yields new insight into classically
forbidden quantum processes. In addition to "physical" values, a realistic
measurement can yield "unphysical" values; we show that in {\it sequences} of
measurements, the "unphysical" values can form a consistent pattern. An
experiment to isolate a particle in a classically forbidden region obtains a
negative value for its kinetic energy. It is the {\it weak value} of kinetic
energy between past and future state vectors.
