A consultant system's main task is to provided helpful advice to the user. Consultant systems should not only find solutions to user problems, but should also inform the user of potential problems with these solutions. Expressing such potential caveats is a difficult process due to the many potential plan failures for each particular plan in a particular planning situation. A commonsense planner, called KIP, Knowledge Intensive Planner, is described. KIP is the planner for the UNIX Consultant system. KIP detect potential plan failures using a new knowledge structure termed a concern. Concerns allow KIP to detects plan failures due to unsatisfied conditions or goal conflict. KIP's concern algorithm also is able to provide information to the expression mechanism regarding potential plan failures. Concern information is passed to the expression mechanism when KIP's selected plan might not work. In this case, KIP passes information regarding both the suggested plan and the potential caveats in that plan to the expression mechanism. This is an efficient approach since KIP must make such decisions in the context of its planning process. A concern's declarative structure makes it easier to express than procedural descriptions of plan failures used by earlier systems.