5.11. My script aborts with an error message, "event not found".

This error is generated by the csh or tcsh shells, not by sed. The exclamation mark (!) is special to csh/tcsh, and if you use it in command-line or shell scripts--even within single quotes--it must be preceded by a backslash. Thus, under the csh/tcsh shell:

       sed '/regex/!d'      # will fail
       sed '/regex/\!d'     # will succeed

The exclamation mark should not be prefixed with a backslash when the script is called from a file, as "-f script.file".