Unfortunately, you'd probably be surprised how often things like this happen in some libraries. I know my mom's caught multiple people on pornographic sites at the library she works at.

Interesting fact, by the way: whenever someone is caught on one of those sites, the librarians need to file a report about it and take pictures of the computer with the site the offending party was on still fully displayed on the screen so it can be shown why that library patron can't have nice things.