Per Norman Harman's request, I'd like to talk about limitations on divination and teleportation spells. The background: a number of GMs complain about these spells because they are "adventure killers", allowing players to solve problems too easily. Teleportation allows adventurers to escape danger; divination allows them to solve a mystery or locate a quest object too quickly, by-passing encounters and puzzles, and in combination with teleportation creates the 15-minute adventure: scry for the location of the necessary object, teleport in, fight the surprised guardians off while securing the macguffin, then teleport out.
Tournament modules with the "teleportation doesn't work" restriction show one proposed solution; mystery-focused modules likewise include a "divination doesn't work" restriction. Proposed in-game solutions include gorgon's blood mixed into mortar, lead-lined walls, restrictions on scrying spell-casters or creatures of higher level, "teleport magnets" that change the destination of teleporters... the list gets pretty elaborate.
I don't really like these solutions.
First, pretty much all of these solutions scream "cheat." How convenient that there just happens to be something that can prevent teleportation or scrying, readily available to the GM now that it's needed, but unmentioned back when the character might have wanted it. The fiat solution of "teleportation/scrying doesn't work" is fine for a tournament module, when players need to focus on the presented scenario. It's not so good for sandbox play, where it seems more like a railroad.
Second, the rationale behind it is, in fact, railroading. The railroad GM wants the players to deal with the arranged encounters, or spend two to five sessions solving a mystery. The players just want to solve the mystery/get the treasure as quickly as possible; if they can do it by divination and teleportation, they will. Why is this wrong? It's not the job of the GM to say "No!" It's the GM's job to make life difficult and interesting for the adventurers. No teleportation bans, just more dangers to teleportation. No divination bans, just more interesting divination results.
Third, it creates an arms race. If high-level NPCs shouldn't exist without protections against scryers and teleporters, because they would all be "taken down" by now, then it also stands to reason that desperate high-level NPCs, trying to take down their enemies, would have researched methods to bypass those same protections... and their opponents would research methods to block the methods that bypass their protections, and so on. You have to stop somewhere; why not stop where there's the most literary and legendary support, with the divination and teleportation spells?
Fourth... well, maybe I should save further discussion for a later post. I'll also cover my own ideas on how I would limit divination and teleportation.