I can recall a few occasions in Northern Ireland back in the day when aircraft were inadvertently overstressed by a combination of weighing errors (describing the load weight in pounds when it should have been kilograms) and pilots being lulled into a false sense of security when lifting loads from hilltops in windy conditions.
Once you get away from the wind it all becomes a bit embarrassing if you haven't got your sums right!
There does seem to be an attitude that 'overgrossing' is an acceptable way of doing business but your aircraft is designed and certified to a certain set of conditions and limits (the RFM graphs) and ANY exceedance of those limits erodes the safety margin for both the short and long term integrity of the aircraft, whether that be engines, rotors, transmission or the structure of the airframe itself.
One of the first things any accident investigator would look at would be the weight and balance of the aircraft to see that it was being operated within the limits of the RFM - if it is outside, that has lots of legal implications, not least insurance invalidation.