Extrusion defects associated with the sharkskin/melt fracture occur at high wall shear rates and stresses. These have been extensively studied and discussed in numerous publications available in the open literature and there will not be part of the present presentation. However, there are other defects encountered in industrial extrusion operations (blown film, flat film), which have received very little attention. These include gels and flow streaks. They are due to a variety of causes which are not easily identifiable. Their common characteristic is that they involve some sort of degradation of the polymer melt. Frequently these defects occur in film extrusion lines which been running problem-free for a long time and may suddenly or gradually appear, especially when a resin with different processability (even minor) is fed to the extruder. While it is possible for gels to be present in the raw material, in this presentation we interested in gels formed in the extruder, due to excessive residence times, mainly because of flawed design of screw channels. They appear as black or brown specks. There is some evidence that the primary cause is the formation of secondary re-circulation flows known as Moffat eddies, in the fluid mechanics literature. Flow lines and streaks are due to flaws in the design of flow paths through adapters, screen changers, melt pipes and dies. Low wall shear rate regions tend to produce streaks of poor optical quality. In single layer extrusion they may be confused with sharkskin. In co-extrusion they might be confused with an interfacial instability known as zig-zag. Computer simulation of flow is used for pinpointing the regions where they originate.