  By numerical simulation, we study the classical magnetoresistance of
two-dimensional electrons in the presence of weak short range scattering. A
critical magnetic field defines the percolation threshold, above which the
longitudinal resistance vanishes. Unexpectedely, just below this threshold we
find a shrp narrow peak, where the resistance may increase 15 times compared to
its zero-field value. By considering the complex topology of the effective
potential landscape for the center of the cyclotron circle, we show that this
phenomenon is related to infinite equipotential lines, which exists only in a
narrow magnetic field interval below the percolation threshold
