Child Protection Act (Rydia Rule):Children 12 and under are exempt from death. They will emerge alive from cataclysms that slaughter hundreds of sturdily-built adults, often with barely a scratch. Further protection is afforded if the catastrophe will orphan the child.
Missing Master Corollary (Sabin Rule):If a fighter's master merely disappeared, you will undoubtedly find him/her at some point in your travels. The master will challenge the student to a duel, after which the student will be taught one final skill that the master had been holding back for years.
Gojira Axiom:Giant monsters capable of leveling cities all have the following traits:
Gigantic teeth and claws, designed, presumably, to eat other giant monsters
Second Law of Fashion:Any character's costume, no matter how skimpy, complicated, or simply outlandish, is always completely suitable to wear when climbing around in caves, hiking across the desert, and slogging through the sewers. It will continue to be completely suitable right afterwards when said character goes to meet the King.
Well, So Much For That:After you have completed your mighty quest to find the object that will save the known universe, it will either a) get lost, b) get stolen, or c) not work.
The Ominous Ring of Land:The classic Ominous Ring of Land is a popular terrain feature that frequently doesn't show up on your world map. Just when you think things are going really well and you've got the Forces of Evil on the run, monsters, demons and mad gods will pour out of the center of the ring and the situation will get ten times worse. The main villain also usually hangs out in one of these after attaining godhood. If there are several Ominous Rings of Land or the entire world map is one big ring, you are just screwed. (See also The Very Definitely Final Dungeon)
Law of NPC Relativity (Magus Rule):Characters can accomplish superhuman physical feats, defeat enemies with one hand tied behind their back and use incredible abilities — until they join your party and you can control them. Then these wonderful powers all vanish, along with most of their hit points. (Commonly related to either Good Is Dumb, Boring and/or Impotent)note For the record, Magus, after whom this rule is alternately named, is a justified example of the trope. His powers were drained by Lavos before he could join you.
Guards! Guards! (or, Lindblum Full Employment Act):Everything will be guarded and gated (elevators, docks, old rickety bridges, random stretches of roadway deep in the forest) except for the stuff that actually needs to be.
Materials Science 101:Gold, silver, and other precious metals make excellent weapons and armor even though in the real world they are too soft and heavy to use for that purpose. In fact, they work so well that nobody ever melts their solid gold suit of armor down into bullion, sells it, and retires to a tropical isle on the proceeds.