Never mess with a Marine's coffee if you want to live. (Season 2, Episode 9)

Rule #23

Never, ever bother Gibbs in interrogation. (Season 4, Episode 10)

Rule #22

It's better to seek forgiveness than ask permission. (Season 3, Episode 4)

Rule #18

If someone thinks they have the upper-hand, break it. (Season 8, Episode 24)

Rule #16

Always work as a team. (Season 5, Episode 5)

Rule #15

Bend the line, don't break it. (Season 11, Episode 4)

Rule #14

Never, ever involve a lawyer. (Season 6, Episode 7; Gibbs is said to have other rules involving lawyers, but according to Tony, this is the "umbrella" one)

Rule #13

Never date a coworker. (Season 1, Episode 15, and referenced on other occasions)

Rule #12

When the job is done, walk away. (Season 6, Episode 24)

Rule #11

Never get personally involved on a case. (Season 7, Episode 21)

Rule #10

Never go anywhere without a knife. (Season 1, Episode 13)

Rule #9

Never assume. (Season 9, Episode 21)

Rule #8

NCIS Quotes

McGee: All right. Well you should probably know that Abby and I used to date?
Bishop: Ew. Like, each other?
McGee: Yeah.
Bishop: Wait - isn't that a violation of rule 12, never date a....
McGee: It was a long time ago. After we'd broken up, one night I went to her lab. Found a scribbled piece paper; a list. Potential boyfriends had to fulfill certain conditions by a pre-arranged date or else, goodbye.
Bishop: Such as.
McGee: Things started off relatively normal: opening the door for her, flowers, putting the seat down. Then around number 8, it gets uh...
Bishop: What?
McGee: Does she know you have these?
Bishop: Does she know you have these?
McGee: Yeah she wasn't happy when she found out.
Bishop: These are all very specific.
McGee: Yeah.
Bishop: These ideas apply to you?
McGee: No those rules weren't in place when we were together. At least I don't think so.
Bishop: What's with the two month cutoff? Abby's sabotaging herself. I've seen stuff like this before. We have to talk to her.

If someone thinks they have the upper-hand, break it. (Season 8, Episode 24)

Rule #16