Sketch interfaces provide more natural interaction than the traditional mouse and palette tool, but can be time consuming to build if they have to be built anew for each new domain. A shape description language, such as the LADDER language we created, can significantly reduce the time necessary to create a sketch interface by enabling automatic generation of the interface from a domain description. However, structural shape descriptions, whether written by users or created automatically by the computer, are frequently over- or under- constrained. We present a technique to debug over- and under-constrained shapes using a novel form of active learning that generates its own suspected near-miss examples. Using this technique we implemented a graphical debugging tool for use by sketch interface developers. Categories and Subject Descriptors: I.2.6 Computing Methodologies, Artificial Intelligence, Learning [Concept learning] I.2.10 Computing Methodologies, Artificial Intelligence, Visio...