There are cases where you may want to have Launchd run a job once, and then have that job removed so it doesn’t run again. This topic came up on the Launch-Dev mailing list not too long ago and some answers were provided. In particular, Quinn the Eskimo said:
“Alternatively, don’t unload the job. Once a job is loaded, it
doesn’t depend on the plist file, so removing the plist file out from
underneath launchd won’t cause you any problems. The job will
continue to exist in launchd (until the system restarts) but it won’t
get run against unless someone specifically invokes it.”