This is the biggest scam in the travel industry

And it's never been more dangerous. Here's why.

Say the word "free," and it's enough to make even the most experienced travelers lose their marbles.

For example, if I offer you 100,000 free bonus miles for signing up for my precious-metal credit card, you might be tempted — even if it's a toxic financial instrument with a ridiculously high annual percentage rate that will take a wrecking ball to your …