Last week, the Federal Reserve spooked markets by preserving the monetary policy status quo. Yet a few central bank watchers were more surprised by a new idea the central bank seemingly suggested: a negative interest rate.

The Fed's closely watched "dot plot" revealed that at least one committee member floated the idea that a fed funds rate below zero might be an appropriate target for the remainder of this year and next.

The forecast is widely thought to be the work of Minneapolis Fed President Narayana Kocherlakota, a non-voting member of the committee who is known for his dovish views. In her press conference last week, Fed Chair Janet Yellen made clear that a negative federal funds rate "was not something that we considered very seriously at all today."

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However, in an environment where prices are persistently low, negative rates mean that businesses and consumers are essentially paid to borrow money, which could serve as an important stimulative tool in times of crisis.

The idea has been floated in the U.K., and tried in the euro zone and Switzerland. Suffice to say, the fact that the concept is being floated in the U.S. is striking to some.

After all, the Fed has spent years noting that its benchmark rate is at the "zero lower bound," which forces it to take other actions in order to stimulate the economy, (i.e. purchase bonds) since rates can't be lowered any further.

Yet what if that bound is not a bound at all, but a Rubicon waiting to be crossed?

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