Even if budgeting isn't something you did much of during your working years, it's extremely important to follow a budget during retirement. As a retiree, you may be a on fixed income that's a combination of savings and Social Security. And you need to make sure you're stretching that income as far as possible.

The nice thing about a budget is that you get to see where your money is being spent. You can also make adjustments to your spending as needed.

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But there's one budgeting mistake retirees risk making. And it could throw you for a major financial loop.

You're probably aware that your retirement budget should include all of your recurring expenses -- costs like housing, healthcare, food, and so forth. But one mistake you risk making in the course of budgeting is failing to build in room for one-off expenses.

During retirement, those could look like different things. You may end up with a large bill on your hands if you end up in the hospital and need to pay your Medicare inpatient deductible (assuming a Medigap plan won't pick up the tab for you). Or, you may end up having to sink a large amount of money into a car or home repair.

These things probably won't happen every month. But they could happen a few times a year. And if you're not accounting for them, your finances could become a mess.

Let's say you have $4,000 a month coming your way between retirement plan withdrawals and Social Security benefits. If your recurring monthly bills eat up that entire $4,000, you won't have any wiggle room for unplanned or one-off expenses.

In this example, a better bet would be to limit your spending on recurring bills to $3,700 or $3,800 a month and allocate the remaining $200 to $300 to one-off costs. That way, if you end up stuck with a $1,500 expense one month or a $1,000 expense another month, you should have money sitting there to cover it.

Now you may be thinking, "But worst case, couldn't I just take extra money out of my savings?" And you could do that. But if you do it too often, you risk depleting your savings in your lifetime.

Spending in retirement isn't limited to monthly bills. Just as surprise expenses can arise while you're working, so too can they creep up during retirement.

The only difference is that during your working years, it may be easier to come up with the money for one-off bills. In retirement, your options may be more limited.

That's why you need to include all of your potential spending in your budget. Accounting for those non-recurring costs could spare you a world of financial stress.

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The 1 Retirement Budgeting Mistake Too Many People Risk Making was originally published by The Motley Fool