`file` on BSD/OSX has this notice for the option '-s':
> Normally, file only attempts to read and determine the type of argument files which stat(2) reports are ordinary files. This prevents problems, because reading special files may have peculiar consequences.
One example that comes to mind is /dev/urandom, which sucks randomy values out of the entropy pool (at least in Linux)—and the pool can be exhausted, or at least it could back in the day, not sure about now. Other possible cases are things in /proc (though unlikely), and particularly stuff like serial ports—where presumably reading could gobble data intended for some drivers or client software.
