Acid stimulation of sour wells, including gas, oil, and water injection, presents special challenges. Add high temperatures and specialty equipment, and sometimes permanent production damage can result. Avoidance of damaging precipitates from unwanted reactions, which can permanently plug a producing interval while providing protection of all production equipment, is sometimes a tenuous balance of design. Precipitation of elemental sulphur, reaction products from incompatible additives used in an acid stimulation fluid, and reprecipitation of iron sulfide scale dissolved during the pumping of acid through corroded tubulars are possible sources of potential permeability restrictions. Corrosion protection to the proper level of weight loss [< 0.0976 to 0.2441 kg/m(2) (0.02 to 0.05 lb/ ft(2)) over the anticipated exposure time period] and no localized surface defects (pitting and stress cracking) both in live and spent acid solutions are essential to protecting the production equipment and treating tubulars. This paper reviews two case studies. The first from Saudi Arabia discusses damage to the outside surface of a coiled tubing string due to acid and H2S corrosion. A Canadian case history describing a mechanical failure of coiled tubing as the result of hydrogen embrittlement is the second case study presented. Guidelines are offered for coiled tubing usage to obtain successful stimulation while controlling risk of tubing failures.