It happened to me a few weeks ago, when a fairly impatient chap bounded past me as I was waiting for the first check for lounge eligibility, but he never actually got to the security scanner before they called him back.

I would assume the FW security gates are programmed to reject anyone not eligible, just as the normal Fast Track ones are for non-eligible status/non-J/F passengers, so presumably if anyone tried to pull a fast one in attempting to enter the F lounge when not entitled they'd be stopped in their tracks fairly sharpish.

The first scan for any guest BPs must also somehow 'code' them to allow access through the automated gates.