From screen's man page:

nethack on|off

Changes the kind of error messages used by screen. When you are familiar with the game "nethack", you may enjoy the nethack‐style messages which will often blur the facts a little, but are much funnier to read. Anyway, standard messages often tend to be unclear as well. This option is only available if screen was compiled with the NETHACK flag defined. The default setting is then determined by the presence of the environment variable $NETHACKOPTIONS.

These error messages are intentionally silly and not given unless you ask for them.