I have never seen any HORRIBLE letters of recommendation (e.g., I do not recommend this student under any circumstances), but I have seen some letters that are very brief and unenthusiastic –
What sort of remarks have you heard about women's clothing? Clothing that is too revealing?
I have only participated on the other end of this process a couple of times, but I definitely saw a gender bias. I heard some remarks about the women applicants that were very catty. Like, "did you see her shoes, she has no fashion sense whatsoever," "she just seems too vanilla," or "she didn't say anything specific to make me think so, but she strikes me as the type that would not be open to feedback." Things like this weren't said about the men (or I just did not hear it). With the men that were ranked lower, there was a pretty clear cut reason (e.g., he doesn't have enough experience with adults, he did not seem enthusiastic about any of the rotations because he asked no questions). However, my experience is limited, so take this with a grain of salt.