The British Proof houses operate on a simple two rule system,
Rule one . The proof can not be wrong make mistakes , their word is final.
Rule two , If it can be shown/proven that beyond all doubt the Proof House is or could be in error , see rule one .
Having dealt with both Proof Houses this joke is pretty close to true . That said errors do occur always have and always will.
Any organisation that has been in existence for so long and undergone so many changes in rules and specifications will have some areas that will always be debatable especially in years pre and post change .New stamps issued prior to a change may and have been used in advance of the official change over date .Imperial to metric for example .
As British Proof Law does not apply in the US it is all academic , guns get altered and rumor has it there several sets of British " proof stamps" in the US .So who knows ?