Written assessment comments are needed to archive feedback and inform decisions. Regrettably, comments are often impoverished, leaving performance-relevant information undocumented. Research has focused on content and supervisor's ability and motivation to write it but has not sufficiently examined how well the undocumented information lends itself to being written as comments. Because missing information threatens the validity of assessment processes, this study examined the performance information that resists being written. Two sequential data collection methods and multiple elicitation techniques were used to triangulate unwritten assessment comments. Between November 2022 and January 2023, physicians in Canada were recruited by email and social media to describe experiences with wanting to convey assessment information but feeling unable to express it in writing. Fifty supervisors shared examples via survey. From January to May 2023, a subset of 13 participants were then interviewed to further explain what information resisted being written and why it seemed impossible to express in writing and to write comments in response to a video prompt or for their own "unwritable" example. Constructivist grounded theory guided data collection and analysis. Not all performance-relevant information was equally writable. Information resisted being written as assessment comments when it would require an essay to be expressed in writing, belonged in a conversation and not in writing, or was potentially irrelevant and unverifiable. In particular, disclosing sensitive information discussed in a feedback conversation required extensive recoding to protect the learner and supervisor-learner relationship. When written comment requests to document performance information are viewed as an act of disclosure, it becomes clear why supervisors may feel compelled to leave some comments unwritten. Although supervisors can be supported in writing better assessment comments, their failure to write invites a reexamination of expectations for documenting feedback and performance information as written comments on assessment forms.