A friend of mine once had telnet'd into their (awful) ISP home router, and started playing with the various commands. Not thinking, they typed "init -h" and the router then promptly stopped routing packets. Turns out the (probably custom-written) "init" automatically sets the runlevel to single-user mode, and they handn't bothered to implement "-h".

For this reason, I always use "man <command>". The prevalence of "--help" has always bothered me since a badly-written command could start doing all sorts of unexpected things, when you just wanted to see the documentation. I've seen all sorts of ISV software not implement --help correctly. "man" always works.