March marked the largest increase in U.S. consumer spending over the last five months. Which means ... the economy's back, baby! People are spending again, and it's only a matter of time before retailers Target (NYSE: TGT) and Best Buy (NYSE: BBY) reap their inevitable and just rewards. Right?

Not so fast
As it turns out, this "recovery" may already be living on borrowed time -- literally. Here's why Target, Best Buy, and other retail-focused investors should get a little antsy:

According to the Commerce Department, consumer spending was up 0.6% in March, but consumers' incomes were up only 0.3%. Where'd the extra money come from? The answer's staring you in the face: It's coming from their savings.

Spurred to responsible savings behavior by the financial crisis two years ago, Americans have already become bored with this whole "responsibility" fad. They're drawing down their checking accounts, cashing out their 401(k)s, and spending the proceeds on pent-up wants, rather than saving for future needs.

Foolish takeaway
I'll be blunt: This cannot last. Unless and until we bring the unemployment rate down, and get more people cashing paychecks and fewer people writing personal checks, the recovery remains at risk.