Abnormal rheological behavior is observed during capillary extrusion with a linear low density polyethylene (LLDPE). The nominal viscosity in the stick-slip flow regime and the gross melt fracture regime increase with a rise in temperature. This is attributed to slippage of the polymer melts. It is generally accepted that the occurrence of slippage can be detected by a Mooney analysis. However, it is found that the Mooney analysis is not applicable to detect the occurrence of slippage in the gross melt fracture regime. This is attributed to the main assumptions in the Mooney analysis being invalid in this regime, mainly due to the presence of a turbulent-like flow pattern. We suggest that failure of the time-temperature superposition principle can be used as an indication of slippage for polymer melts.