Just punched your best friend in the face? Put some ice on it and he'll be fine. Just wrecked his car? No prob, he's got insurance. Slept with his ex-wife/girlfriend? Whoah momma, you're in for a world of hurt!
Even among the closest Heterosexual Life-Partners, there are some lines you just do not cross, and one of them's dating, sleeping with, or otherwise getting involved with a woman your bro has broken up with. If a guy breaks this rule, expect it to lead to incredible hostility in the friendship, lasting ... oh, about thirty minutes or so.
A Truth in Television, as dating/sleeping with/any romantic involvement with a friend's ex (without asking first) can be interpreted as a disrespectful stab to the back (as well as to the heart).
See its sub-trope Bros Before Hoes. Compare to My Sister Is Off Limits, when the conflict is over dating a bro's sibling.
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In a Miller Lite series of commercials called "Man Law" delved in many other important subjects like "is a person allowed to stick a finger in a beer bottle to carry it" and "how many beer cans can you take home from a friend's party", and in one a panel of men debate how long you have to wait before making a move on a friend's ex-girlfriend. Opinions range from "forever" to "two Saturdays, or a new hair-do, whichever comes first." Just for the record, the final word given in said argument is "six months".
In an amusing example of defictionalization (see the How I Met Your Mother example below), Barney's 'The Bro Code' has been published as an actual book. Hilarity ensues, naturally.
Live Action TV
On Arrow, this is a standing problem between childhood best friends Oliver Queen and Tommy Merlyn, both of whom are in love with Laurel Lance. Recognizing that Laurel is still in love with Oliver, Tommy breaks up with her. Oliver then entreats Tommy to reconsider and get back with her, since his own life as the Hood would make having a relationship with Laurel very difficult. Immediately after having this conversation, Oliver violates the Bro Code and betrays Tommy by seeking out Laurel and having sex with her. Tommy, on his way to come see her, observes through the window.
How I Met Your Mother is the Trope Namer. Barney has a long list of rules governing how male friends (or "bros") are to act towards each other when there's a woman involved, which he claims was created by his 18th century ancestor Barnabus. (It's worth noting that The Bro Code has subsequently been defictionalized; it's available at bookstores around the world, with the e-book read by Neil Patrick Harris.) This particular rule is only one of many (specifically, Article 150), but it forms the focus for the episode "The Goat" and several episodes that follow, after Barney sleeps with Robin, Ted's ex-girlfriend.
Although, Barney was not in violation of the Bro Code, since he asked Ted's permission before actively pursuing Robin.
Played for Laughs when Ross dates Chandler's ex-girlfriend Janice, and is worried he'll be angry. Chandler makes a big deal of forgiving him and makes him promise to do the same. (As he's secretly dating Monica at the time and is worried about Ross's reaction).
They had a similar issue before, when Ross kisses Chandler's MOTHER (who was played by Morgan Fairchild).
Played more seriously when Joey falls in love with Rachel and agonizes over Ross's reaction.
Chandler falls hard for Joey's girlfriend Kathy. Would be normally ok, but Joey is serious about her too...
In Two and a Half Men, Charlie definitely did mind when Alan was seeing Chelsea (mainly because he is still hung up about her). Charlie initially forced Alan to not go anywhere near Chelsea's birthday party, he changed his mind later but he felt that he had to out of fear that Chelsea will get mad at him for knowing why Alan didn't show up to her party.
In Power Rangers Operation Overdrive, Will makes a fakeFace-Heel Turn and joins Kamdor and Miratrix. Dax is especially upset... that Will's with his ex. (Miratrix and Dax dated before she revealed she was running a Honey Trap.) While the girls exchange a look, Dax and Mack explain with "Man law!" (check the Miller Light commertial above)
Wilder talks about this in The Latest Buzz, calling it one of the 'guy rules,' specifically the rule 'pals before gals.'
In That '70s Show, Kelso was quite upset when he found out that his best friend, Hyde, started seeing his ex-girlfriend, Jackie.
In the Psych episode "High Top Fade Out", Shawn, Gus, Henry, Tony, and Joon throw out different definitions of "The Code", (almost?) all of which relate to sleeping with your friend's girlfriend or relative.
In the Seinfeld episode "The Ex-Girlfriend", Jerry starts dating George's ex-girlfriend, Marlene, and worries that he'll be upset. However, when he eventually tells it to George, it turns out that he doesn't care.
The backwards episode involved a variation with Jerry setting George up with an ex had never actually slept with. Unfortunately, Jerry ends up sleeping with her just before actually setting them up. He tries to keep it from George, but its gets out and he's pretty upset over it.
Saturday Night Live parodied this in a sketch involving two bros who get into an argument over a woman, hash things out, and manage to reconcile, all by using nothing but various inflections of the word "bro".
In Neighbours when Todd started dating Josh's ex Phoebe.
An episode of Saved by the Bell had Slater convince Zack to go out with one of his old flames in order to convince his current girlfriends that he was over her. Slater then gets jealous when said Old Flame and Zack really hit it off.
There was also the episode where Zack kissed Lisa, the object of Screech's affections. That didn't go over well either.
Smallville: Clark and Whitney start off as enemies, but at the end of season one, they reconcile their differences and parted on friendly terms. Clark refuses to go after Lana until after she formally breaks things off with Whitney.
In season five, Lex gets involved with Lana very quickly after she broke up with Clark, and soon progressed to sex and then marriage. It destroyed what is left of Clark and Lex's friendship.
In season six, it's lampshaded when, in an effort to start up a friendship with him, Jimmy affirms to Clark that he is his "bro", not his "foe".
In True Blood, Jason and Jessica start sleeping together after she breaks up with Hoyt. Hoyt angrily says Jason broke the "Bro Code".
Mentioned in Disgaea 4. Fuka briefly considers pursuing Valvatorez as a "reasonable compromise" to finding her Knight in Shining Armor, but decides against it because it would be a total violation of The Bro Code, and both Vulcanus and Fenrich would get all up in her face for it.
The reason Tails is Sonic's sidekick instead of a female character.
One of the endings in Catherine has Vincent breaking up with Katherine and Johnny deciding to make a move on her. He was upset at first but easily gets over it.
Lou: Well if you just wanna talk, why don't we talk about Eddie sleeping with my ex-wife?
Chief Wiggum: I thought the divorce was final.
Lou: When is a divorce ever final?!
In Happy New Year, Charlie Brown! Linus ends up dancing with Charlie Brown's crush, The Little Red-haired Girl. Of course Linus claims he didn't know where Charlie went, never mind he saw him go outside to do his homework. And Linus wonders why the Great Pumpkin passes him by.
In real life, it's fairly concrete as "current significant other, mother and sister off limits, exes are a complicated issue." Beyond that it gets pretty nebulous.
In the UK it's normally called "the Guy Code", and beyond "bros before hos" and not sleeping with your mate's girl, the rest comes off as more as a subconscious knowledge that men share and know automatically and without question when it has been violated.
Its most famous violation and discussion came when footballer John Terry slept with national teammate Wayne Bridge's ex. There was much notation that while there was no technical legal harm, the Man Code had clearly been broken.