The magnetoresistance of a narrow single quantum well is spectacularly different from the usual behavior. At filling factors 2 / 3 and 3 / 5 we observe large and sharp maxima in the longitudinal resistance instead of the expected minima. The peak value of the resistance exceeds those of the surrounding magnetic field regions by a factor of up to 3. The formation of the maxima takes place on very large time scales which suggests a close relation with nuclear spins. We discuss the properties of the observed maxima due to a formation of domains of different electronic states.