MICHAEL MUKASEY: Trump, Cohen, and Attorney-Client Privilege: The protection has limits, but is it worth testing them over a possible campaign-finance offense?

After anthrax spores killed five people, infected 17 others, and showed up in envelopes mailed to U.S. senators and media organizations in 2001, the current special counsel, then director of the Federal Bureau of Investigation, spent years chasing and destroying the reputation of a microbiologist named Steven Hatfill, zealous in the belief that Mr. Hatfill was the guilty party. Another zealot, James Comey, then deputy attorney general, said he was “absolutely certain” no mistake had been made.

After Mr. Hatfill was exonerated—he received more than $5.5 million in damages from the government—Mr. Mueller then decided that another microbiologist, Bruce Ivins, was the culprit. When Ivins committed suicide, Mr. Mueller pronounced the case closed. A subsequent investigation by the National Academy of Sciences suggests Ivins too was innocent.

Mr. Mueller is not a bad man, nor is Mr. Comey. It’s just that both show particular confidence when making mistakes, which makes one grateful for safeguards like the attorney-client privilege.