On Tuesday, mortgage rates fell to their lowest levels in 4 years. It happened because the Fed said it would “employ all available tools” to resuscitate the economy.

On Wednesday, however, the markets had second thoughts.

After considering the long-term implications of a near-zero percent Fed Funds Rate and the cumulative cost of government intervention to-date, suddenly, traders grew fearful that U.S. government action would devalue the dollar and lead to inflation — the enemy of low mortgage rates.

As a result, mortgage markets unwound.

At first, the exit was a slow and orderly. Then, without warning, investors began a full-on sprint for the exits. By the end of the day, mortgage rates were higher by as much as a half-percent. Nearly all of Tuesday’s big gains were erased.

In hindsight, the reversal Wednesday wasn’t all that surprising — it’s the same trading pattern we’ve seen twice already this year. The first time was after the Fed’s “surprise” rate cut in January, and the second time was after the federal takeover of Fannie Mae and Freddie Mac in September.

Sharp rate drops tend to be followed by immediate bounce-backs, it seems.

But, unfortunately, not every would-be refinancing homeowner saw the increase coming. While those that locked at the first opportunity to save money are sitting pretty today, the rest that “waited for rates to go lower” are likely kicking themselves about it.

Going forward, mortgage rates may fall, or they may not. We can’t possibly know. But we’ve now seen the pattern 3 times now — when mortgage rates plunge like they did Tuesday, they rarely stay that low for long. When you find a rate you like, get in and get locked as soon as possible.

Sleeping on it for even one night may end up costing you dearly.