In San Francisco, Portland, and certain places in Los Angeles, there is a significant number of homeless people who make use of the library. I don't begrudge them the access to books, internet, learning, and a quiet place, but there is a small contingent who are mentally ill and/or disruptive and cause problems for other patrons. Cf. “Why we can't have nice things”

This is most likely true of any large city, but I don't have any first-hand experience beyond those three cities.