The first is that too often their management decided to expand by acquisition at home and abroad and into areas where they had no real expertise.
Generally they were forced into humiliating retreat at huge cost to shareholders and reputations. The privatised utilities are the prime examples.
The second sin was going to war with their regulator, notwithstanding that the regulatory function had been created to stop private sector monopolies throwing their weight about. Doing down the regulator became an obsession at Railtrack, British Gas and BT. They all lost. The British do not care much for regulators but they trust them more than the motives of the management of these former nationalised industries.
Royal Mail now shows alarming signs that it is becoming obsessed with its regulator. The past few months have seen a series of attacks on the regulator whose job it is to open the UK market up to competition and to ensure the Royal Mail does not wield too much power. There have been rows about postal charges, post office investment, co-operation with competitors - the list goes on.
Presumably the plan is to pressure the regulator by whipping up a media storm. Or perhaps the target is government. If it senses the public is siding with Royal Mail, it might rein in the regulator. Either way the intention is to neutralise his office.
Perhaps it is inevitable that a business which is a monopoly will act this way when that monopoly is broken, but it is still a mistake. Sir Ian Vallance at BT, Sir Denis Rooke at British Gas and Gerald Corbett at Railtrack all came to regret their battles with the regulator. Royal Mail has a uniquely favourable rating among British consumers in spite of the grumbles about first-class post. It would be tragic to see that dissipated.