Had this happened with me then i would have called them, told about this mistake and requested to take back additional amount which has been wrongly credited. If operator is helpful and supportive then we should also be absolutely fair , honest towards them. Unfortunately people criticise companies for shortcomings but when some error benefits them most prefer to stay silent and not take correct steps.
Also even if some company is following unfriendly strategy then is reacting with viciousness right approach? For me putting efforts for improvement is way forward and if things do not fall in track then opting out is best. They caused 100 Rs. Loss so i will accept Rs. 200 if it comes by luck in my account seems to cause more friction than settling scores