A 'BIGOT' list is a list of personnel possessing appropriate security clearance and who are cleared to know details of a particular operation, or other sensitive information.

More information on the Wikipedia page [1].

There were various 'almost leaks' and breaches - of which the most peculiar occurred with various of the crosswords in the London Daily Telegraph newspaper, where a number of codewords were given as answers to clues. The crossword setter, a schoolmaster, was arrested, but on investigation found to be innocent: it was eventually revealed that he had asked his pupils to provide ideas for crossword clues, and one of them had been listening to soldiers' conversations.

There are a number of articles on the subject, including the National Geographic article [2], the Historic UK page [3], and point 2 on [4]: most repeat the same information. The Wikipedia page is [5].

See also D-Day carrier pigeon cipher