Millions of seniors today count on Social Security to pay the bills once they stop working, and chances are, it will be an important income source for you one day as well. But the program is loaded with rules and quirks that could throw your retirement off course if you're not careful. Here are a few "gotchas" to keep on your radar.

1. Less replacement income than expected

Many people assume that Social Security will mostly replace their paychecks at their jobs. But that's just false. If you're an average earner, you can expect Social Security to take the place of about 40% of your wages. If you're a higher earner, your benefits will replace an even smaller percentage.

Loose pile of Social Security cards

Image source: Getty Images.

That's a problem because most seniors need roughly 70% to 80% of their pre-retirement income to live comfortably once they stop working. This is not an exact formula, and if you intend to live frugally, you may very well manage to get by on less. But you should know that when it comes to replacement income, 40% is the target you're looking at.

2. Taxes on benefits

If Social Security is your only source of retirement income, or even your primary source, then you may be able to avoid taxes on it. But if you have outside income (money from a savings plan, investments, or rental income from a property you own), then you should expect the IRS to take a chunk of your benefits.

To see whether you'll pay federal taxes on Social Security, add up your provisional income, which is your non-Social Security income plus 50% of your annual benefit. Keep in mind that certain types of income, like Roth IRA withdrawals, don't count toward provisional income.

If your total falls between $25,000 and $34,000 and you're a single tax filer, or between $32,000 and $44,000 and you're  married filing jointly, you could face taxes on up to 50% of your benefits. Beyond these thresholds, you could be looking at taxes on up to 85% of your benefits.

On top of federal taxes, you may be taxed on Social Security income at the state level. There are 13 states that tax benefits to different degrees.

3. Stingy raises

Social Security beneficiaries are entitled to a yearly cost-of-living adjustment (COLA). But these raises aren't always much to write home about. Over the past 12 years, there were three years when seniors didn't get a raise at all. And for the past two years, COLAs have averaged just 1.45%. Because Social Security does such a poor job keeping up with inflation, it's important to have an income source other than these benefits -- ideally, a source with the potential to grow over time.

Some surprises are pleasant, like the retirement party your colleagues throw for you. Others, however, can be dangerous, and the above items fall into that category. The solution? Read up (a lot) on Social Security in the course of planning for your senior years. Knowing how the program works could spare you a world of financial stress later in life.