Rules

Rule #1: Never let suspects stay together. {Yankee White (1)}
Rule #2: Always wear gloves at a crime scene. {Yankee White (1)}
Rule #3: Don't believe what you are told. Always double check. {Yankee White (1)}
Rule #3: Never be unreachable {Deception (59)}
Rule #7: Always be specific when you lie. {Reveille (23)}
Rule #8: Never take anything for granted. {Probie (56)}
Rule #9: Never go anywhere without your knife. One Shot, One Kill (13)}
Rule #12: Never date a co-worker. {Enigma (15):}
Rule #18: It's better to seek forgiveness than ask permission. {Silver Wars (50)}
Rule #23: Never mess with a Marine's coffee if you want to live. {Forced Entry (32)}
unnumbered: Never say you're sorry. {Yankee White (1)}

In Switch (51), during a conversation in the elevator, Gibbs tells David that there are "about 50" rules. In Deception (59), DiNozzo and Ziva discuss rule #3, which they say is "never be unreachable.". In Yankee White (1) we are told rule #3 is "Don't believe what you are told. Always double check"