Everyone gets excited at the thought of our potential tax refund. Almost everyone is guilty of making plans for all that extra money – but what happens if you spend beyond your means?

Let me tell you about my friend Jason. Jason (who wished not to use his real name) had big plans for his anticipated tax refund. When he finally had that money in his bank account, he was going to pay off his credit card balance.

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You see, he had recently treated himself to a brand new, 80-inch television. When he bought it, he didn’t have the money to cover it. But it was on sale, and he figured a smart shopper would jump on that deal. So he put it on his credit card and said that he’d pay it all off once he got what was supposed to be his hefty refund check.

A Common Tax Refund Mistake

Tax time came, and Jason crunched the numbers. Then, assuming there must be a mistake, he crunched them again — and again, and again, and again. Finally, he made an appointment with a tax accountant to find out what was going on.

Turns out, Jason had completely overestimated the amount he’d get back from the government. He was counting on receiving a lump sum of around $3,000 (after all, the average American’s tax refund is right around there). Unfortunately for Jason, what he actually got back was a measly $200.

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Now Jason had a real problem on his hands: He had purchased an item that he couldn’t afford, and it was too late to return it. He was stuck searching his couch cushions over the next few months, scraping together the money to make ends meet and pay for a television that he shouldn’t have bought in the first place.

Common Money Mistakes

I know what you’re thinking: Jason is an idiot. You’d never do something so stupid. But chances are, you’ve fallen into this very same trap at least a few times before, even if not quite on that scale.

There’s one critical money mistake that far too many people make, even if they’re financially savvy: counting on money that they don’t have.

Your checking account is fine, you tell yourself, because you have a birthday check from your grandparents on the way. Or maybe you go ahead and overspend just a little bit because you remember that a friend still owes you money for concert tickets that you bought. Either way, you made the same blunder that Jason fell victim to: You counted money that wasn’t in your wallet or bank account yet.

Banking on Inheritance Money

Gifts and I-owe-yous aside, this can also happen on a larger scale. Many people count on receiving a large inheritance when a loved one passes away.

This expectation is far more common among younger people, which can be particularly dangerous, since they’re just beginning to learn how to successfully manage their own finances. Forty-four percent of American adults between the ages of 18 and 34 expect their relatives to leave them money or valuables. And as it turns out, they aren’t just counting on a couple hundred dollars or grandma’s favorite quilt. No, their expectations are pretty high.

A 2007 Gallup poll revealed that 39 percent of those who think that they will receive an inheritance expect the total value given to them to be under $100,000. Meanwhile, “A little less than one-quarter of respondents think they will receive between $100,000 and $249,999, and about 3 in 10 expect it will be more than that.”

But between inheritance taxes and, in some cases, the pricey family disputes that an inheritance inspires, most people don’t end up receiving what they thought they would. And if and when they do get it, most Americans only manage to save half of it (if they’re lucky).

The Bottom Line

So if there’s one important thing to remember, it’s this: No money is guaranteed unless it’s already in your bank account. Not your tax refund, not your birthday check, and not your inheritance.

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Sure, your grandfather or great-aunt may leave a sizable inheritance or send you a check when your birthday rolls around. But don’t count on it. When it comes to your finances, only count the money that’s a sure thing. That is to say, the money that you already have in the bank.