Ten years ago, investors knew that technology would change the world, and they were right. But you can be 100% right about the future and end up with zero to show for it if you overpay in the first place.

On March 10, 2000, the Nasdaq Composite index closed at a record of 5048.62, up 24% since the beginning of the year, after an 86% gain in 1999. And many were calling for the Nasdaq to hit 6000 within a year or two.

It...