scary but it happens SRT!
there was once a story of a military air uplift from hawaii i think and the marines had chartered a world airways type of outfit for use of their dc8.
the civvie "loadsheet guy" calculated the weight based on the standard 75kg per pax when indeed each marine( not light chaps by any standard) had 100pounds of gear. there were about 150 of them.
they just made take off before the end of the runway helped by the fact they were at sea level.
for aircraft which don't have load sensing i think the solution is to have a weight recording/sensing loading conveyer on the airside monitored by security. they could still be bribed but it will be reduced by virtue of not being in contact with the pax.