Non portable behaviour is a trap

Most often, man pages are not forthcoming enough about what is and isn't portable.
It's common for me to learn, say, a command line flag and even build it into muscle memory,
only to later find out it isn't portable when I run shellcheck on a script.
This has created a sad state of affairs where online man pages are better than native ones,
at least if you're writing a script you mean to be portable.

Implementations of POSIX commands should be *narrowly* POSIX compliant.