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 Inevitables, which are divine golems with the duty to uphold laws and correct the broken ones. There's a story of one named "Obligatum" who is 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. 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.
• 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 Arcadia got annexed 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.
• The Starks from Game of Thrones.

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. 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.