Lawful Stupid

Extreme lawful neutral who think they're good. The most annoying character in your party, who will insist everyone has to give up 10% of the loot to orphans, or else he'll make your life hell. He'll stop a chase scene because he has to chastise someone that was jaywalking. Less annoying that stupid good, who will refuse to allow your party to hurt the world devouring elder evil out of their love for all things, living or otherwise. Less funny than chaotic stupid, who will do anything as long as it's batshit insane, like randomly pissing on the king's head. Less stupid than stupid evil, who will go out of his way to look for dogs to kick. Less facepalm inducing than neutral stupid/stupd neutral who will kill both the employer and the villan, and his party, and generally everyone he meets to maintain some sort of vague cosmic balance even though it's fairly obvious that their actions are screwing up any balance there is to be had.

Examples of Lawful Stupid

• The Inevitables, which are divine golems with the duty to uphold and correct broken laws. There's a story of one named "Obligatum" who is the BBEG in a campaign because some mages summoned a cthulhubeast named Pandorym on purpose to get a fair bargain with the gods and imprisoned it instead of finishing the ritual to letting it loose so that it wouldn't destroy the universe before they were ready, but the gods just smited the stupid wizards the instant they were done imprisoning Pandorymn 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 it's (the universe's) inhabitants can muster if it's (Pandorymn) 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.

• Most Spees Mareens. 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 chapters.

How to avoid it while playing lawful good

Required Reading: Discworld by Terry Pratchett, in praticular anything having to do with the Witches of Lancre or the Ankh Morpork city watch.