In the last few years, there has been a growing interest in the areas of telemanipulation, task planning, and, in a more general sense, in efficient interfaces to robot systems. In this context, telerobotics studies how control can be shared between a human operator and an intelligent robot controller. This research investigates the problems associated with manipulation and, to a certain extent, programming errors in a shared operator/computer control of a robot system. The principle is to trace all actions, at run time, to provide on-line detection and recovery of errors. A world model is constructed and maintained for the purpose of predicting the effects of actions and signaling errors when the actual outcome of an action differs from its required effect. Default reasoning is used extensively to speed up processing and compensate for the high cost of sensing. After a task planner has dealt with the general organisation of the program, the system presented here has the responsibility of coping with variations of the real world to attain the desired goal with the given plan. A test case, overhead power line maintenance, demonstrates the functioning of the system and, although the work is based on this particular context, the scheme described comprises a generic "substrate" which deals with common basic robot actions such as move and grasp and is supplemented by task and environment specific knowledge such as which parts can be mated, sizes, and weights. This part of the system is static for a given task and a good portion of it, the substrate, is valid for a wide range of tasks.