In his autobiography Clinging to the Wreckage the playwright and former barrister John Mortimer tells how when he was practising at the Bar in divorce cases he acted for a husband whose wife's lawyers repeatedly alleged that the husband had a concealed source of income which could be used to provide additional maintenance payments. The man repeatedly denied this until after persistent probing he at last admitted that he had such a source but claimed that it was covered by the Official Secrets Act and could therefore not be disclosed. Urged to confide in his counsel he finally revealed that he was a part-time hangman at executions. Mortimer a lifelong opponent of capital punishment refused to act for him any further.
Such extreme ethical problems are unlikely to confront advisers today but people do sometimes own up to concealed...