The normal slow crack growth behaviour of polyethylenes consists of a ductile-brittle transition where the transition to brittle fracture occurs below a critical stress and after a critical time. The curves of stress vs. failure time depend on temperature, but all curves can be shifted vertically and horizontally by well established shift functions. It has been observed that there are some polyethylenes whose stress vs. time to failure curves do not shift in the normal way and in some cases the time for brittle fracture does not change with temperature. SEM micrographs do not reveal any difference between the fibrillated fractured surfaces from the normal and abnormal specimens. Whereas the normal slow crack growth behaviour has been attributed to the disentanglement of the molecules by sliding without chain scission, it is proposed that the abnormal fracture behaviour may involve a disentanglement mechanism that consists of a combination of chain sliding and scission.