Sometimes, surprises in life can be pleasant, like if a friend sends you flowers on your birthday or a rainy day forecast turns sunny. But other surprises aren't as fun -- namely, some of the financial ones you might encounter during retirement. Here are three to keep on your radar so your finances aren't thrown off-course.

1. A lower Social Security benefit than expected

Many people go into retirement assuming that Social Security will provide enough income to cover most, if not all, of their expenses. In reality, the average senior this year collects a mere $1,543 a month.

Rather than rely too heavily on Social Security, work on boosting your personal savings so you have income on hand to supplement those benefits. If you have access to a 401(k) plan at work, it pays to sign up, especially if you're eligible for any type of employer match. And if you don't have a 401(k), fear not -- you can build a nice nest egg for yourself in an IRA instead.

It also helps to get an estimate of your monthly Social Security benefit ahead of retirement so you know what number to expect. You can access that information in your annual earnings statements, which you can get a hold of by setting up a Social Security account.

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2. A property tax hike

Many seniors enter retirement owning their homes outright. But even if you manage to shake your mortgage payments in time for retirement, your housing costs could still eat up a lot of your income. That's because property taxes have a tendency to rise over time -- even during periods when home values fall.

A good way to protect yourself against increasing property taxes is to pad your savings. Depending on where your live and the value of your home, you may have a year when your property tax increase well outpaces the raise you get from Social Security, so the more money you have in savings, the more peace of mind you'll buy yourself.

3. Services Medicare won't cover

Many seniors are shocked to discover that Medicare won't cover all of their health-related needs. If you stick to original Medicare, you'll be responsible to cover the cost of dental care, eye exams, and hearing aids if you come to need them. And while Medicare Advantage, an alternative to original Medicare, will generally pick up the tab for these costs, you may find that an Advantage plan is too limiting for you.

The solution? If you're eligible to put money into a health savings account, or HSA, do your best to max one out while you can. That way, you'll have money set aside for future healthcare expenses so that when Medicare falls short, you won't be left in the lurch.

Now one thing you should know is that you can only put money into an HSA if you're enrolled in a high-deductible health insurance plan, the definition of which changes on an annual basis. But it pays to see if you're eligible, because like 401(k)s and IRAs, HSA offer a world of tax benefits.

Surprises can keep life interesting, but the above surprises could wreck your retirement and cause you a world of undue stress at a time in your life when you deserve better. Now that you know to look out for these pitfalls, you can take steps to prevent them from ruining your senior years.