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.
Categories: cond-mat quant-ph