jw0001:

I've never taken a pre-employment polygraph for a federal agency so I can't speak from experience, but other posters have stated that there's nothing wrong asking for a retest even if the chances are small; also you can apply to other agencies,  since it's been suggested a failed polygraph at one agency does not necessarily mean you'll fail at a different agency (but you will probably be asked if you've been previously polygraphed.)

Regards, Evan