We find anomalous magnetoresistance peaks in a narrow Hall conductor, which are positioned in the quantum Hall plateau regions rather than in the plateau transition regions. Such peculiar behaviour can be explained quantitatively by using a model based on the edge current picture, in which the Landau levels crossing the Fermi energy are different in the conducting segments of the current and voltage probes of the sample. The Landau level onset is attributed to the nonuniform distribution of the carrier density.