12.5KG and 50KG specified cylinders should be filled with 12.5kg and 50kg LPG GAS respectively. Overfilling can cause problems, as it would reduce the ullage expansion space. Overfilling, accidental overfilling or overfilling due to inaccurate equipment (i.e. the scale being used in the filling process not being calibrated or accurate) will have the same results as purposely overfilling a cylinder. The effects of overfilling can have catastrophic effects as described below.
A cylinder filled to 100% of its capacity would vent through its safety device before the cylinder warms to room temperature. In summary, never overfill your cylinder, on purpose or accidentally. The effects of overfilling coupled with the effects of increasing temperature, will greatly increase the probability that something catastrophic could happen to human or property. Do not take the risk, do not overfill your cylinder. That GAS plant or vendor who always claim to have filled more than the recommended is not doing you any good in overfilling your cylinder rather endangering your life.