When specifying both --length and --bytes in either long or short form, the value of --length will be chosen while the value specified by --bytes will be discarded silently. While I don't think anybody would intentionally run into it, having a warning or error just in case is something that would help improve UX.

Note: I'm doing a class assignment regarding creating tests (from scratch) for an open-source command line program, and I chose hexyl for it. You might have a couple more of these on the way, depending on what the assignment brings to light :)