Chances are nothing serious will happen... at first. The aircraft is built to withstand a certain load factor at the maximum floor loading.
Then you have a hard landing or hit severe turbulence, getting close to the limiting load factor... and the floor collapses. Floorboards or seat tracks/seat track support structure fail and you end up with aircraft structure, cargo and underfloor aircraft systems entangled in one another.
The last item would be the main concern. You may find hydraulic lines, environment system ducting, packs, avionics, fuel lines and control cables there, none of which are likely to react well to having floor beams crash down upon them.
In short - don't.
The result I've actually seen is cracked floorboards, with no further consequences. Not due to flying with floor load limits outside of tolerances, but due to exceeding them while loading. Expensive.
On a sidenote, try calculating the floor loading under the heels of a pair of high-heeled shoes... ouch! Then try telling a 50 kg woman that you have to place spreaders in the aisle before allowing her to board... double ouch!
