I think I can recall someone posting on the forums after getting into trouble with FCC Revenue Protection when trying to board a FCC train which was about to leave Kings Cross during the "rush hour." The ticket was from south of London ( Surrey / Sussex / Kent) to a station in FCC land and the restriction did allow the journey to be undertaken during the rush hour from Kings Cross. The RPI disagreed and problems ensued. Someone more knowledgeable may want to expand on this and offer sounder advice as I cannot remember the full story.
I can see why this happens. FCC set some fares which have standard "Southeastern" offpeak restrictions (C4) from "their" stations to Southeastern stations, e.g. St Albans Central to Paddock Wood SAC-PDW. This allows travel northbound from London in the evening peak.
But the equivalent ticket to London Terminals has an FCC offpeak restriction (TN) which does NOT permit travel northbound from London in the evening peak.
It's interesting, because the Paddock Wood tickets works out cheaper if you travel after the morning peak, but return in the evening peak, because you can buy a CDR SAC-PDW (£21.20) rather than an SDR SAC-U12 (£22.90)
There is a similar situation at Liverpool Street where there are large (incorrect) signs telling intending passengers that "offpeak tickets are not valid between xxxx and xxxx" (in the evening peak), which is only true on CERTAIN ticket types and very few through London tickets.