"Why is my subject search for 'Gender expression' turning up so many titles on aerodynamics?" I was wondering just now, until I realized the most likely explanation was some catalog update process had done a pattern search-and-replace on "Drag" as a subject without checking the results.

(Seen in a HathiTrust subject search. Most likely not their fault, as they take records from partner libraries pretty much as they get them.)

@JMarkOckerbloom I have a perhaps related anecdote. Several years ago, I was trying to organize a billiards party, but trying to search for materials for it failed because the search engine decided that billiards and pool are synonyms so I must want to search for pool parties. And adding "-pool" didn't help since a lot of sites that talk about billiards also talk about pool.