The National Labor Relations Board issued a public version of a previously unreleased directive ordering regional officials to begin prosecuting unions for “negligent” behavior that the board used to view as harmless error.

The board first issued the directive in an internal memo that was obtained and reported on by Bloomberg Law. NLRB general counsel Peter Robb’s Oct. 24 directive seems to curtail that order slightly and could mean less aggressive litigation against unions for careless or unprofessional behavior, although...