The incorporation of extrinsic defects into mercuric iodide substrates during detector fabrication can be extremely detrimental to device performance. In particular, extrinsic defects can act as trapping and recombination centers, and they can reduce charge collection efficiencies, decrease gt product, or contribute to polarization effects in nuclear detectors. In this paper we present results of processing, photoluminescence, and electromigration experiments that clearly show that extrinsic defects can be incorporated in mercuric iodide during detector fabrication. By observing the luminescence features characteristic of Cu and Ag in mercuric iodide, we show that both these materials are taken up by mercuric iodide crystals during etching with KI if the etching solution is contaminated with these elements. The migration of material from contacts into the crystal, as shown by resistance measurements, is also presented. We infer from this work that other defects which are detrimental to device performance may also be incorporated in mercuric iodide if insufficient care is taken during device fabrication. Suggestions are therefore made as to some of the precautions that must be taken in order to realize the highest quality detectors.