We report experimental observations and matching numerical simulation for the time-dependent start-up flow of two molten polyethylenes (PEs) within a slit entry and exit geometry. For the case of a low density polyethylene (LDPE), an unexpected transient, birefringence "stress fang" was observed downstream of the slit exit. The stress fang consisted of a localized region of stress concentration. The stress fang, however, was not observed for a linear low density polyethylene (LLDPE) sample subjected to the same processing condition. A matching time-dependent numerical simulation of the flow is also presented. Using a split Lagrangian-Eulerian method for simulating transient viscoelastic flow with the multimode pom-pom constitutive equation, the general features of the stress fangs were predicted for the LDPE. In addition, the simulation did not predict stress fangs for the LLDPE. The paper demonstrates that for this particular case the pom-pom model can successfully discriminate the complex flow behavior of different PEs, and shows that the presence (or otherwise) of a stress fang is sensitive to the particular theology of the polymer that arises from long chain branching.