Fads, by definition, fade. But fading is too gentle a term for what happened yesterday to shares of rubber sandal-maker Crocs (CROX).

The company issued a profit warning Monday after the bell and the stock fell more than 40% yesterday. Management blamed weakening economic conditions for its poor outlook, but analysts question the long-term viability of the brand.

Crocs are easy to criticize. The rubber shoes are by most accounts ugly as sin and, throughout much of the world, completely impractical for six months of the year. They're great for kids, but in most family units ten-year-olds make precious few purchasing decisions. They are however, sturdy.

That's precisely the problem. Crocs are a one-time purchase. Their rugged rubber frame is built to last, hurting the prospect of repeat business. Crocs should take a hint from Apple (AAPL), whose popular iPods are designed to wear out every couple of years. Failing that, a five dollar umbrella salesmen outside Grand Central Station provide a reliable primer in built-in obsolescence.