Tolkein skated around a few things, but surprisingly few. I noticed that putting the Ring of Power on in later books drew the immediate attention of the evil forces, especially close to Mordor. This was not true in "The Hobbit," where Bilbo made extensive use of the Ring, including wearing it for many weeks at a stretch while trapped in the elves' stronghold, with nothing untoward occurring. I suppose there's a good reason for this.

Terry Pratchett, in his books, commented a number of times on why more people didn't use magic routinely when it was obviously a real phenomenon. The short answer is that wizards and witches knew damn well what happened when magic got into the general population, and that it was a resource best kept largely unused.

The current crop of wizards in the Discworld series primarily hang out at the university and take it easy. Some books, like "Sourcery," describe the hazards when wizards get ambitous.

In the most recent book, "Going Postal," the plot required an instantaneous communications device operated by magic. In a book which deals mostly with the complex methods of postal and mechanical semaphore communication that are recent developments on the Discworld, you can see where this would present a problem. Pratchett goes to great lengths to explain why you can't have one of these devices in every home, but it's a close shave.

The fact of the matter is that you cannot have any kind of a decent book plot in a world where the suspension of reality is a simple matter, and no decent fantasy writer includes magic without a huge number of strings tied to it.

In a similar vein, Larry Niven wrote a series of mystery stories taking place in the future ("Gil the Arm") and has discussed how carefully one has to tread when laying the ground rules to keep technology from becoming a universal "deus ex machina."

Star Trek has this kind problem in its first series. Every show that added a new gimmick added another load of problems to later shows.

The transporter, for example, was a brilliant idea that saved the writers a lot of money and time in the shows, but all later shows had to come up with many elaborate reasons why they couldn't simply beam the protagonists out of a given situation.