The dirty secret about the so-called fiscal cliff is that many taxpayers have already toppled over the edge.

While much airtime is being devoted to the Bush-era tax cuts that are about to expire, the reality is that some critical credits went belly up at the end of last year.

Homeowners could get hit especially hard on their 2012 tax returns, losing both the ability to write off mortgage insurance premiums and energy-efficiency improvements to their homes, tax experts said. Teachers can't write off school supplies purchased out of pocket for this year. Congress could restore the credits with action before the end of the year.

Paychecks in 2013 will be smaller right out of the gate when the payroll tax cut ends at the end of this year. And a final kick in the pants could come from a new schedule change: Early filers, who tend to get about $1,000 more than the average taxpayer, will have to wait longer to get their refunds. The earliest filing date has moved back to Jan. 22, from Jan. 17 last year.

Here are 10 ways you'll feel tax bill sticker shock starting Jan. 1, unless Congress makes changes:

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  • Residential Energy-Efficiency Credits

    No tax breaks for being green in 2012. Homeowner investments in energy-efficient double-pane windows or high-efficiency refrigerators, <a href="http://www.energytaxincentives.org/">don't get any tax benefit, and that could add to tax bills</a>. More than 43.5 million Americans have filed and received this benefit, with an average reduction in tax liability of $765.84, according to H&R Block.

  • Mortgage Insurance Premiums

    Bad news for homeowners: They cannot write off mortgage insurance premiums on 2012 tax returns. Congress can act to reauthorize these deductions retroactively to Jan. 1, 2012, and extend them through the end of 2013, but that would cost the government $1.3 billion over the next decade, the <a href="http://articles.latimes.com/2012/oct/07/business/la-fi-harney-20121007">Los Angeles Times reports</a>.

  • Adoption Credit

    Taxpayers who have out-of-pocket adoption expenses or who adopted a child with special needs can only claim the adoption credit to the extent of their tax liability. While the portion of the credit not taken into account in 2012, up to $12,650 per child, is carried forward to future years, the benefit is no longer fully refundable, according to tax experts.

  • Alternative Minimum Tax

    If the alternative minimum tax legislation is not retroactively patched for 2012, current law could result in an increased tax liability for up to 34 million Americans. According to a study by the Tax Institute at H&R Block, an average family making $85,000 with children in college could see their tax liability soar from a $1,056 refund to owing $1,400.

  • American Opportunity Tax Credit

    Tuition bills will be higher starting on Jan. 1 because families will lose the $2,500 American Opportunity Tax Credit, which ends in 2012 unless Congress takes action. More than 2.4 million Americans claimed this deduction in 2009, resulting in a combined decrease in taxable income of $5.4 billion, according to tax experts.

  • Payroll Tax Credit

    Paychecks will be smaller starting Jan. 1, 2013. An American making $50,000, for example, will lose $80 in monthly pay after the credit ends. The temporary credit also has lowered the amount workers contributed to Social Security by 2 percent.

  • Educator Expense Deduction

    Teachers lose their $250 maximum deduction on expenses related to buying school supplies. This credit expired at the end of 2011, and teachers won't be able to claim this benefit on their 2012 taxes unless Congress takes action. In 2009, more than 3.8 million teachers claimed this benefit for a combined deduction of $9.7 billion, according to H&R Block.

  • Sales Tax As An Itemized Deduction

    Taxpayers will no longer have the option of claiming an itemized deduction for state sales tax in lieu of state income tax. This expiration will have a greater impact on taxpayers who reside in a state with sales tax, but no income tax, including Alaska, Florida, Texas, Nevada, Washington, South Dakota, and Wyoming.

  • IRA Retirement Funds

    Taxpayers over age 70½ no longer have the option of directing their income from an IRA distribution to a charitable organization. Starting this year, older taxpayers must include the distribution in income and claim a charitable deduction, resulting in a potentially higher tax bracket and a need to itemize instead of claiming the standard deduction, according to H&R Block.

  • Later refunds

    As if losing all those tax credits was not bad enough, the earliest date to file a 2012 tax return electronically has moved back to Jan. 22, 2013. As the IRS has indicated that refunds could take as long as 21 days to process this year, a refund in January to cover Christmas credit card payments, winter heating bills, or rent is unlikely.

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