The Social Security Administration estimates that 12% of male recipients and 15% of female recipients count on their benefits to provide 90% or more of their income. And among elderly beneficiaries, 37% of men and 42% of women get 50% or more of their total income from Social Security.

As such, it's fair to label Social Security as a critical income source for retirees. This applies even to those who have a decent amount of savings in an IRA or 401(k) plan.

But one long-standing Social Security rule puts many seniors in a position each year where they're unable to enjoy their benefits in full. And if you're not aware of it, it could result in a huge financial shock once your retirement rolls around.

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You may be taxed on your Social Security income

Many seniors are surprised to learn that Social Security benefits can be subject to taxes at the federal level (some states tax benefits as well, but that's a separate discussion). Now notice how we've said "can be." Benefits aren't automatically taxable. Whether they are, however, is a matter of provisional income.

Provisional income is essentially your non-Social Security income plus 50% of the amount you receive in benefits each year. So if your non-Social Security income amounts to $20,000 and you also receive $18,000 in benefits each year, your provisional income is $29,000.

The problem with provisional income is that it doesn't take a lot of it to cross the threshold at which Social Security benefits become taxable. In fact, if you're single with a provisional income of $25,000 to $34,000, you can be taxed on up to 50% of your Social Security benefits. If that number exceeds $34,000, you can be taxed on up to 85% of your benefits.

As you might imagine, these thresholds are slightly higher for married couples filing joint tax returns. In that case, if you have a provisional income of $32,000 to $44,000, you can be taxed on up to 50% of your benefits. Beyond $44,000, you're potentially looking at taxes on up to 85% of your benefits.

Now if you're thinking, "Wow, these thresholds are low," it's because they were established decades ago and haven't been updated since. But unfortunately, you could get stuck paying taxes on some of your Social Security income even if your total income isn't all that high.

Avoid an unpleasant surprise

Social Security benefits are only one of several income sources that can be taxable in retirement, so it's important to sit down with a financial planner and get a better sense of what your tax burden might look like. At the same time, there may be steps you can take to keep more of your Social Security benefits.

One option is to keep your retirement savings in a Roth IRA. These accounts offer the benefit of tax-free withdrawals that don't count toward provisional income, so that's a good way to limit the amount of Social Security income you lose to the IRS.

Ideally, lawmakers will one day revisit the aforementioned thresholds to help them align with living costs today. But since those thresholds haven't changed in decades, there's no reason to assume this will be a priority. As such, even lower-income seniors have to brace for the possibility that some of their Social Security income will be lost to taxes.