It is surprising how much irrelevant detail some letters of recommendation include, and just how much this can hurt the candidate. Consider this excerpt from a letter written in the medical field (4), in which the letter author goes into an inappropriate level of detail about a candidate’s medical problems:
Her last years in my laboratory were impacted by serious health problems that have fortunately gone away—she had really debilitating problems with a herniated disk that apparently was a paraneoplastic phenomenon that went away once an early carcinoma of the left ovary was identified and removed.
Here the detail is not just absurdly clinical in context, but discriminatory and potentially damaging. A generous reader might interpret that the writer is well-meaning—intending to explain, perhaps, a long number of years in a lab or a weak publication record—but many would interpret that the writer actually intends to do harm. Whatever the case here, clearly the detail presented is highly irrelevant. Several studies suggest that letters often reveal more about the idiosyncrasies of the writer than the characteristics of the applicant (5,6,7), and that letter readers certainly do attend to irrelevancies when reading, in some cases over-relying on them when making decisions (8). These irrelevancies might be about either the letter writer or candidate, and either way harm can be done. When writing letters, avoid providing details that are unimportant to the selection process and may only distract or puzzle the audience. Common irrelevancies in letters include commentary about the student’s personal life or health, digressive discussions of ideology, events, or research beyond the audience’s needs, vague examples or claims that simply go unexplained, or a focus on the letter writer’s personality or beliefs that ultimately doesn't matter to the reader anyway.