The worst I experienced was in Singapore. Kids running around like it was a playground. Or teenage couples using it as a place to date. The last one is a big wtf head scratcher to me.
In both my current and previous US city, downtown libraries function as de facto daytime homeless shelters. On the one hand this is good, as people in unfortunate circumstances can get Internet access, apply for jobs, and retain access to email. On the other hand some of those people browse porn, play music, and watch videos without headphones on the library computers. Others wander in off the street to have loud conversations with the front desk staff.
Where I live now, this feels like the library’s main purpose. Either as a cause or a consequence, library staff seem to see themselves as social workers first, activists second, and librarians third, as reflected in the library’s public services and calendar of events.
Around ten years ago I didn't have the money for an office so would work a few hours each day in the 'quiet' section of our local library. I could sometimes go a whole 5 minutes without wanting to murder someone.