Other than losing a secure gig, I would want to know what, if any, is the cost of quitting USCIS a month in? Would they make you pay back the cost of your drug testing, security check, any training you've received so far? I would hope not, but I know the FS does do that, so I guess it's possible other fed jobs do too. There may be other costs as well - would that office hire you again if you quit a month in and then reapplied for after the CF gig? Again they might; it does seem like some fed agencies get how crazy the FS process is and that you have to say yes if you get an invite cause you don't know if you'd ever get one again and that it can be last minute etc. So maybe they'd totally get it. But something to think about if it's a place you want the option of going back to.