We are told that long-term capital gains and qualified dividends are taxed at the rate of 0% if you are in the 10%-15% brackets. But this income increases your MAGI for purposes of calculating taxable Social Security or Railroad Retirement benefits. So if you have $1,000 in qualified dividends, which you expect to be totally tax free due to the 0% bracket, you could end up paying tax on as much as $850 at your "normal" ordinary income rate!
An afterthought - there is another way that tax-exempt municipal bond interest can end up biting you in the arse. The amount of exempt interest reported on Form 1040, or Form 1040A, Line 8b is added to your AGI to determine if you will be charged a higher Medicare Part B premium.