As a librarian, it is often necessary to create rules to address unruly patron behavior, which can often exhibit itself in new and surprising ways.
One might surmise that, at one point in time, saxophone usage was a problem in the library of the Stockholm School of Economics in Stockholm, Sweden. But, Altas Obscura informs us, it was just a prank by an anonymous person many years ago.
The library staff appreciated the humor and decided to keep the sign in place. In fact, it even put out a public notice informing patrons that the saxophone problem had been resolved.