Many seniors rely heavily on Social Security to cover their living expenses. And unless you enter retirement with a heaping pile of cash in an IRA or 401(k) plan, you might have a similar experience.

But the last thing you want to do is get caught off guard on the Social Security front and struggle financially because of it. Here are three less-than-pleasant Social Security surprises that could wreck your retirement if you don't prepare for them.

1. Your benefits won't replace your entire paycheck

Many people assume that once they retire, they'll collect a Social Security benefit that's the equivalent of their former paycheck. Not so.

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If you're an average earner, you can expect Social Security to replace about 40% of your pre-retirement income. If you're an above-average earner, those benefits will replace an even smaller percentage.

Meanwhile, most seniors need more like 70% to 80% of their former earnings to live comfortably. As such, it's a poor choice to rely on Social Security as your sole income source.

A better bet, in fact, is to get an estimate of what your future Social Security benefit might look like so you can see how much of an income gap you'll be facing. That might help you better map out some savings goals.

You can get an estimate of your retirement benefit by accessing your annual earnings statement, which will come to you in the mail if you're 60 or older or is other available once you create an account on the Social Security Administration's website.

2. Filing early slashes your benefits for life

You're entitled to your full monthly Social Security benefit at full retirement age, which, depending on when you were born, is either 66, 67, or 66 and a certain number of months. However, you're allowed to file for benefits once you reach age 62.

Claiming benefits at 62 will leave you with less money each month. But contrary to what you may have been led to believe, that reduction isn't temporary. Or to put it another way, if you file for Social Security at 62, your monthly benefit won't be bumped up to its full amount once you reach full retirement age. Rather, you should expect the initial benefit you collect to be the same benefit you receive for life.

3. Social Security income is taxable

Many seniors are shocked to learn that Social Security benefits can be taxable. Whether that applies to you, however, will depend on your total income, as well as the state in which you retire, as some states impose their own taxes on benefits, while the majority do not.

A good way to avoid having to pay taxes on Social Security is to house your retirement savings in a Roth IRA. Since Roth IRA distributions don't count as taxable income, they won't be calculated in the formula that's used to determine whether you'll be liable for Social Security taxes at the federal level.

Meanwhile, reading up on the states that tax Social Security can help you determine what to expect in that regard. Keep in mind, though, that some of the states that tax benefits also offer exemptions for lower-income seniors.

Don't be in the dark

The more you know about Social Security, the better prepared you'll be for retirement. Keep reading up on how Social Security works so you can approach your senior years with the confidence you deserve.