Thoughtful mentoring can shape and challenge a beginning teacher's practice in educative ways, especially when the novice is well prepared and adopts the stance of a learner. What responsibility does a mentor have when the novice performs at the edge of acceptable practice? Drawing on interview and observation data collected for a national study of new teacher induction, this article explores how a well-supported mentor routinely missed opportunities to address difficulties faced by three novices. Through the construct of professional accountability, the authors argue for a more sophisticated approach to mentoring that blends assistance with standards-based assessment, as found in recent reform proposals. As the pool of new teachers shrinks, the phenomenon of teaching at the boundary of acceptable practice is likely to be exacerbated. This analysis raises timely questions about mentors' professional obligations to new teachers and the public they serve.