Tales of the Questor is filled with this trope and subversions, and just reading the comic would be faster than listing every case. Some noteworthy examples include taking on a rat-king on his own with nearly suicidal results, freeing a thief he believed would be punished remarkably severely, feeding said thief after she tried to steal from him, and being polite and friendly to humans he had little reason to trust. When Quentin reveals himself to the villagers to help fight the evil Fey lord, his honorable behavior he displayed at the farmer's home comes into play when that farmer speaks up and tells the crowd that he trusts the Racoonan hero. Even more recently, attempting to draw the attention of said evil Fey lord to protect a bunch of humans earned him Three Wishes.
However, Honor Before Reason is nowhere to be found when he makes those three wishes. He — as a narrator — tells us that even one carefully-worded wish could ruin a fae. When he's done making his three, the evil Fae Lord is utterly ruined. Then again, perhaps he is showing honor — by protecting the mortal realm by turning their nemesis into the fae version of a penniless vagabond, especially when he could have wished for all his grand quest items to allow him to return home in triumph.
It may not be immediately obvious, but most of his Honor Before Reason behavior is attributable to his own naïveté. Taking on a rat-king alone was a matter of being in a hopeless scenario. If he ran, the shadow rats would have overwhelmed and devoured him anyway. He helped the thief in question less because of honor and more because he's a soft touch. As to wishing for the Fae Lord to retrieve all the quest items for him, that was a little bit above the Fae's pay grade (they're powerful, not omnipotent or omniscient). Phrasing the wishes just right to avoid a backlash would have required a platoon of lawyers, and even if the Fae had granted the wish he would still have been left with a very powerful and very ANGRY Fae Princeling ready to squash him like a bug. His three wishes were phrased so as to minimize the damage the Fae Princeling could cause. He is largely oblivious till after the fact what a perfect storm of bankruptcy his wishes have caused the Fae Lord in question.
Honor and Reason go hand in hand when he takes on his current quest. He acts with Honor by fulfilling an ancient contract to save a village, fully knowing he may never be able to return home. He acts with equal Reason—it's his hometown, and if he turns this quest down, he will never be able to return home as his family is in the exact same predicament as everyone else. Even if he dies without completing his quest, his village is protected.
And yet again, when he takes on the mission to kill a dragon that had been terrorizing the countryside. After the guardsmen sent to assist him abandon him in the middle of the night, he decides to press on... despite having little-to-no supplies and only Sam and a disgraced squire (with a possibly haunted suit of self-motivating magic armor) as back up. Though this time it's heavily implied that it's as much about Quentyn's ego as it is about keeping his word.
Homestuck is an interesting case of this. The troll society of Alternia allows mindless killing of any on a lower bloodcaste than oneself. It encourages it, even. Along with that, revenge is encouraged, as is pretty much anything. The subversion, and in being so, the played-straight (by human standards) example is Karkat, who, despite copious swearing, has not once hurt another troll.
Double Subverted. Karkat is a mutant and therefore has the lowest bloodcaste, meaning he cannot really kill anyone, culture be damned. He also planned on joining the military when he was older and respects Alternian culture. A better example within the same comic would be Tavros, who used his Psychic Powers to play through the entirety of SGRUB without killing a single Imp.
The Order of the Stick: Lord Soon of the Sapphire Guard swore an oath of non-interference regarding the Snarl's Gates, other than his own. This was a good idea at the time, to prevent infighting from spoiling old friendships. However, all the paladins of the Guard still consider themselves bound by this oath, even though those to whom it was sworn are (probably) all dead, and seizing the Gates before the Big Bad does is the key to saving the multiverse. Nevertheless, the oath takes precedence over the paladins' drive to oppose evil wherever it be found. This forces Lord Shojo to get creative, and hire the title party to investigate the Gates instead. Ironically, at least one other Scribble member thought Soon would break his oath, and booby trapped the location he gave for his Gate in an act of spite. Double irony: he was the only one that didn't break it.
On the other hand, this led to O-Chul being able to completely avoid compromising ANYTHING about the other gates. This is lampshaded by Redcloak, who remarks with frustration that it is absurd for generations of paladins to wilfully sabotage their own ability to perform their duties, all for a silly promise. A (literal) lampshade is then promptly hung around the lampshade itself.
No longer true. A leader of the paladins eventually offers to help the Order of the Stick in their quest, if only by covering one of the remaining gates when the main characters go to find the other. He explains that with their Gate destroyed, the oaths that bound them are dissolved.
Durkon declares he and Hilgya must part because they must do their duty  — followed by Manly Tears.
The entirety of the qualified regulars (except for Parakewl) in Tower of God one by one decide to help Baam and Lahel take the Guardian's test, even though they've known each other only for a month and expected to fight each other, and even though that specific test is harder than the usual course. Special mention goes to Hatsu, who is the most immediate and most vocal proponent of supporting Baam, and Koon, who by pretending to be against it riles most up to follow Hatsu.
The Knights of La-Shoar have a strict policy on anything that goes against "Natural Law", policies that have become defacto law in their territory - at the top of that list is magic. Any magic, from healing magic to offensive spells to charmed items. Not only does this put their kingdom at a disadvantage (Every other major power makes open use of magic), but they know it. But refuse to change their ways at all.
Lady of War Bernadette jumped through every ridiculous hoop The Knights put up to test her "suitability" to be one of their numbers. They had to be sure she wasn't "cheating" or just getting lucky when challenging other knights. (As if her taking down an Artifact of Doom-wielding psycho who'd carved through their ranks wasn't proof enough.). This has been Bernadette's life dream. And just when the elder Knights formally ask Bernadette to join them... she turns them down. She chose to come out of the closet as Maytag's lover, rather than be forced to deny her as a knight. (Homosexuals also being against "Natural Law") Note that Bernadette and Maytag were very much on the down low before Bernadette's moment and Maytag would've been perfectly happy to keep it that way.
In TwoKinds, this trope is the Eastern Basitin hat, to the point that they're biologically tuned to accept and obey orders, even clearly self-destructive ones. (Keith's ability to disobey is considered "proof" that he's "broken and unfit".)
Hell, as one of the few who are able to disobey orders, Keith tries to off himself from the crushing guilt. It should be noted that not every Eastern Basitin is happy about this urge and can deeply regret following questionable orders.
Villainous example: The Wizard's Apprentice in El Goonish Shive. He swore to his mentor and God that he would kill all of the Dewitchery Diamond's spawn, which previously had all been monsters. Now that he's discovered that Ellen is not a monster but instead an Opposite-Sex Clone who has done nothing wrong, well, he feels really bad about it, but he takes his oaths very seriously.
In Castlevania RPG, Katrina has been harassing Shaft (one of Dracula's lieutenants), convinced his take over of a villiage is part of some master plan of villainy (he was elected mayor through no trickery on his part). In exasperation, Shaft removes the Cat Girl curse he'd placed on her years ago, thinking that would shut her up. Instead, it made her angrier, since she was convinced she had to "earn" the curse's removal through good deeds and demanded Shaft re-curse her. He does - again, just to shut her up.
Sir Muir in Harkovast pretty much personifies this trope.
Big Ears from Goblins qualifies, as it is usual for paladins. He would throw himself "into the fires of hell" if he thinks it's the right thing to do, but fortunately he can be reasoned with by his companions.
Avery, Sisko's player from D&DS9 informs the DM that the Borg's roll was a Critical Hit, despite it not being in his interest to do so.
In Earthsong, this drives Nanashi's fighting style and her way of life. She detests cowardly tactics, and never uses her native human soulstone power of telepathy and mind control. Not only does this lead to several incidences where she manually investigates matters that she could clear up with a mental scan, it leaves her vulnerable to a human controlled by Beluosus later on. She can only partially resist his powers, which ultimately forces her to make a Heroic Sacrifice.