Lawful Stupid

Obligatum VII, the definition of Lawful Stupid.

Lawful Stupid is gamer slang (it is derived from the Dungeons & Dragons alignment system, but can easily be applied to characters in any role-playing game -- in fact, it can be applied to characters in any medium, or even real life) for a specific way of playing a Lawful Good character, usually a Paladin. It is characterized by lack of common sense, following the rules arbitrarily without actually understanding them and just generally being an annoying prick. He's that guy who will stop a chase scene because he has to chastise someone that was jaywalking. Lawful Stupid players are one of the main reasons (along with asshole DMs) why people dislike the Paladin class.

It can also be a jab at the fact that Intelligence is a common dump stat for Paladins, since thanks to their MAD they require high Charisma and Wisdom (the traditional dump stats of combat classes).

Compares to Chaotic Stupid, Stupid Evil, Stupid Good, and Stupid Neutral. There really are a lot of ways to be stupid in fantasy games, aren't there?

Examples of Lawful Stupid

• The Inevitable, which are divine golem-like beings with the duty to uphold laws and correct the broken ones. There's a story of one named "Obligatum VII" (the seventh in its line because six time prior people had the common sense to stop him) who trying to free the BBEG in a campaign. The story goes that some mages summoned an eldritch abomination named Pandorym to get a fair bargain with the gods. The wizards imprisoned Pandorym instead of finishing the ritual to let it loose so that it wouldn't destroy the universe before they were ready, but the gods just smote the stupid wizards the instant they were done imprisoning Pandorym so he's stuck. Well, Obligatum is here to set things right, and make sure that poor, imprisoned omnicidal maniac gets the freedom it was promised to carry out its goal, which through some warped sense of honor it is willing to do. An omnicidal maniac who is powerful enough to destroy the entire universe regardless of whatever feeble resistance the universe's inhabitants can muster if Pandorym's mind and body were ever to be reunited. How exactly this does not bring him into conflict with another type of Inevitable, the Varakhut, whose job it is to prevent deicide is a whole other box of worms.
• The Harmonium from Planescape. The Harmonium believes that peace is a better end than war. [...] If it takes thumping heads to spread the truth, well, the Harmonium's ready to thump heads. Sure, there may not be peace right away, but every time the Harmonium gets rid of an enemy, the multiverse is that much closer to the universal harmony it was meant to have. This attitude is how the third layer of Arcadia shifted into Mechanus, and the gods of Arcadia had to start over. Whoops.
• The stereotypical Space Marine. Stealth is cowardice, frontal assaults are the only way to go. On the occasion they do utilize tactics like stealth, feints, and flanking, it's all to help the frontal assault succeed rather than the other way around. Retreating is never an option, even if it's to gain more cover. Some will never field psykers, ignore xenos, and some won't even cooperate with other Space Marine Chapters. A special case being Leandros.
• The Starks from Game of Thrones. When Ned Stark finds out that Joffrey and his siblings are incest born bastards, he does the most asinine thing possible and tells Cersei, instead of going to Robert directly. He also tells his daughters of his plan, which causes Sansa to blab to everybody. His son Robb Stark has even more fuck ups, namely executing one of his top generals when he should have kept him around, failing to communicate with Edmure (though Edmure is incompetent), and blatantly breaking his promise to Walder Frey because he felt bad he screwed some other chick and decided to marry her in order to keep their honor intact. Admittedly, he is still a kid in the novel.
• Rorschach from the Watchmen comicbook is this considering his utter devotion to principles. It's best illustrated in his quote "No. Not even in the face of Armageddon. Never compromise." It's later shown that he does not mean that metaphorically. He's an interesting example because he is perhaps the least 'good' of the main characters (with the exception of Comedian maybe) who has no problem with killing even when it's probably not necessary, and with absolutely no empathy. But at the same time he has his code and he sticks to it, even when it is directly stupid of him to do so. Not compromising is an absolute hallmark of lawful stupid.

How to avoid it while playing lawful good

Required Reading: Discworld by Terry Pratchett, in particular anything having to do with the Witches of Lancre or the Ankh Morpork City Watch, especially Sam Vimes and Carrot Ironfoundersson. Carrot might not be very savvy when it comes to subtlety, but he is very much a good man, if a bit odd and literal at times. Some folks would argue that Vimes is Chaotic Good rather than Lawful Good, but fighting over alignments is for the alignment page -- the point is that he and Carrot are decidedly not Lawful Stupid.