I suspect its origin is from residual phosphorus trichloride in step 2, which reacts with thionyl chloride in step 4 to produce PCl5, and then reacts with Hydrogen Fluoride in step 5.Based on the rotten egg and pungent odor, I strongly suspect that Thionyl chloride was used in this process (as opposed to another chlorinating agent such as Phosphorus Pentachloride).