Have you ever walked into a grocery store to find a brand-name product on sale for less than the generic? Or driven to a fast-food joint to find cheeseburgers selling for less than hamburgers?

These are rarities, sure, but they happen. It's not every day that value bucks conventional wisdom, but it's not without precedent. Yet you probably never expected to see tax-free money market funds yielding more than their taxable counterparts -- before taxes.

IMoneyNet reports the unlikely this week. The average tax-free fund is yielding 1.22%, while the typical taxable money market vehicle is producing a yield of just 1.21%.

Face it. The payout levels are pathetic. The Fed slashed rates by 50 basis points yesterday, and they will probably follow the limbo stick lower in the weeks to come. No one views tax-free funds as a get-rich-quick scheme, but has short-term fixed income become so jaded that tax-free municipals can't command a yield discount due to their tax-advantaged features? Or, in a more likely scenario, have yields fallen to the point where expense ratios matter more than ever, and some taxable funds are skimming a bit too much off the top?

The popular, low-cost Vanguard Prime Money Market Fund (Mutual Fund: VMMXX) is yielding 1.47% right now. While its efficient 0.33% expense ratio has typically kept its yield 30 basis points or so above the industry average, now that the payouts are getting this low, the differences on a percentage basis are significant. The Vanguard Tax-Exempt Money Market Fund (Mutual Fund: VMSXX), which charges just 0.18% in annual expenses, is yielding 1.65%.

Can it be? Are tax shelters becoming the house built of bricks? It may just be a temporary blip, but keep an eye on this if you're parking idle funds. Most brokerage houses offer taxable and tax-free funds, so look into which option best suits you.

When the better parking spot also comes with a taxman bypass option, it might be time to make the switch.