Sketching provides a natural and compact means for a user to outline a plan for a high-level objective. Previous work on plan sketching required that sketches be valid, meaning that there be at least one legal completion of the sketch relative to predefined planning knowledge. This paper addresses the problem of plan sketch interpretation when the validity assumption does not hold. We present a formal framework for robust plan sketching that defines key concepts and algorithms for interpreting and repairing plan sketches with respect to two classes of problem: violated applicability conditions and extraneous actions. We also describe a mixed-initiative implementation of this framework that supports a user and the system working collaboratively to refine a plan sketch to a satisfactory solution.