The Institute for Safety Technology (ISTec) has been involved in software reliability research and assessment for more than 25 years. The emphasis has been placed on applications in nuclear power plants, especially on software based instrumentation and control (I&C) systems important to safety. In this context potential sources of weakness in ANSI C programs, which may be error prone have been investigated from a practical point of view. The paper describes essential results of this study; it gives not a complete list of weakness points, but essential examples are shown. From the potential sources of weakness recommendations are derived to improve the C code also in view of software modification and maintenance, which is important because of the long life-time of I&C systems in nuclear power plants.