Something I've never quite understood... (Why a Ring?)

[The Saucepan Man]

surely someone as intelligent and skilled as the Dark Lord should have had the foresight to build in a fail-safe.

He did. Well, two actually. First, no one could willingly destroy it. And secondly, it was imbued with part of his will so that it had a will of his own and, if lost, could take steps to make its way back to him. This it could do either by slipping off its bearer's finger (as it did with Gollum), or by forcing its bearer (or those around him *coughBoromircough*) to succumb to it.

The only drawback, absent divine intervention (doh! ) would be if someone more powerful than Sauron could use it against him. And the only one capable of doing so was Gandalf (and conceivably Saruman), who wasn't around when he made it. And even then, they would have become corrupted, and in effect, mastered by the Ring (and therefore a part of Sauron's will).