The "lfs rmfid" does not print anything useful or exit with a non-zero error code when passed bad arguments:

# lfs rmfid --help
# lfs rmfid foo
# echo $?
0

It would make sense to handle "--help" to print the usage message, and should print an error and the usage message if a bad filesystem name/mountpoint argument is given or if a bad FID is passed (though it should continue to process all remaining FIDs on the command-line and exit with an error at the end).