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6 Tales of Tax-Time Blunders: 'The Major Mistake I'll Never Make Again'

03/26/2015 01:27 pm ET | Updated May 26, 2015

It's officially tax season -- that time of year when most people aim to sail through the onerous process as quickly as possible.

But there's a major downside to rushing.

Since it's probably safe to say that many of us haven't read the U.S. tax code verbatim, there's quite a lot of room for error when it comes to correctly filling out forms and filing your return.

You can take my word for it.

When I landed my first full-time job out of college, I made the mistake of claiming three allowances on my W-4: one since I only had one job, another because no one else could claim me as a dependent, and a third because I was "head of household."

Yep, I assumed that since I was living alone in a studio apartment, I was the head of my own household. (Wasn't I?!)

As a result, my employer didn't take out nearly enough in taxes -- which, to my dismay, resulted in a whopping $2,000 tax bill.

Luckily, I had an emergency fund and several boxes of mac and cheese on hand to afford the unexpected bill from Uncle Sam. But since then, I've been careful to withhold more than enough in taxes to avoid such surprises in the future.

Hungry for more taxing horror stories? Read on!

Tax Offender Tale #1: 'I Didn't Report All My Income'
"I used to earn my living as an independent contractor, doing IT work for Fortune 500 companies, which meant I had to report my income using 1099 forms issued by each of my clients.

I've been doing my own taxes since I was 15, and I've always been very organized when it comes to keeping up with my paperwork. But I'm human -- and mistakes do happen.

Case in point: One year I did contract work for seven different companies but forgot to input one of my 1099s when I did my taxes. Of course, that company didn't forget me, and filed their forms with the IRS.

When I was audited three years later, I got a huge surprise: a bill for back taxes to the tune of $3,000. Even worse? I wasn't earning much at that point, so I couldn't spare the cash to pay off the debt in a lump sum.

Fortunately, I was able to negotiate a settlement with the IRS for 20 cents on the dollar, and they were amenable to a payment plan.

But suffice it to say, I learned my lesson, and now I'm even more careful to confirm that I have all my documents on hand before I file."

-- Carlos Peláez, 34, environmental nonprofit cofounder, Walnut, Calif.

Tax Offender Tale #2: 'I DIY-ed My Taxes -- Even Though I Shouldn't Have'
"In 2014 I was using software to file my own taxes when I accidentally input $2,781 of deductible expenses under the wrong category heading.

As a result, I didn't receive any tax breaks. In fact, the mistake triggered a tax bill of $1,543, instead of the refund I should have received.

I didn't even realize I'd made an error until a few weeks after I filed. When my boyfriend asked how much of a refund I was expecting, he was confused when I informed him I had to pay. So he reviewed my return -- and discovered the error.

Fortunately, there's a long grace period to make corrections and file an amended 1040X document -- but it's a painful process. Once I finally submit the form, I'll be anxiously awaiting my refund.

From now on, I'm not relying on my own know-how to complete my taxes -- I'll make sure to ask someone to review my return before submitting it!"

-- Elizabeth Billings*, 30, editor, New York City

Tax Offender Tale #3: 'I Misinterpreted a Tricky, Lesser-Known Tax Law'
"In 2008 I made a huge tax mistake that cost me considerably. It wasn't a blunder in computing or filing my taxes, but rather a misunderstanding of an obscure tax law.

When I accepted a one-year position in Iraq as a government contractor from June 2008 to June 2009, my company told me that my salary would be exempt from federal taxes -- yet that wasn't quite true.

The exemption only applies to a single tax year, which is January to December. But since I was gone over the course of two tax years, I should have been paying federal taxes the whole time. How I wish I'd researched that more thoroughly!

I first recognized the error when I was completing my 2008 tax return, and the software computed a $15,000 bill -- I was expecting closer to $1,000.

Shocked, I started researching tax publications and forums, realizing the mistake -- and that my 2009 return would be subject to roughly the same amount. Unfortunately, there was nothing I could do about it, and ended up coughing up a total of about $30,000, which the IRS allowed me to pay over two years.

Ever since, I've made it a priority to educate other government contractors and help them avoid the same horrible fate."

-- Glen Wiggy, 49, government analyst, Monument, Colo.

Tax Offender Tale #4: 'I Trusted a Careless Tax Preparer'
"Last year my now wife and I hired a professional to prepare each of our tax returns, who told us we owed the government more than $7,000 combined.

We couldn't believe it.

My wife works as a freelancer and diligently pays her estimated quarterly taxes, while I withhold as much as possible from my own paycheck to make sure this kind of thing doesn't happen.

At first, we were resigned to paying the bill -- but we couldn't shake the feeling that something wasn't quite right. So we asked both of our dads to review the returns.

It turns out that the preparer made a huge error inputting our property taxes, which created a balloon payment of about $5,000 -- something both our parents caught.

While it might seem a bit juvenile to have your parents double-check your tax return as 30-somethings, we're glad we did. Needless to say, we're not working with that tax preparer anymore, and have hired someone my dad has vetted and trusts."

-- Greg Simmonds*, 32, software engineer, Santa Barbara, Calif.

Tax Offender Tale #5: 'I Took an Unsubstantiated Deduction'
"Shortly after I graduated from college in 2009, I started repaying my student loans.

I knew the interest payments were tax-deductible, so when I began preparing my 2010 return, I tallied up what I'd paid and entered the sum.

Not long after, I received a letter from the IRS notifying me that the student loan interest deduction I claimed was unsubstantiated.

What I learned was that in order to deduct student loan interest, you have to file something called a W9-S form with your loan servicer, so that they have your correct taxpayer identification number. I didn't know to do that, so the amount of interest I paid over the year differed from what my loan servicer reported to the IRS.

Unfortunately, this was a pretty expensive mistake. While I'd initially received a refund of $1,674, the IRS reversed it and sent me a bill for $1,159. I also had to pay $73 in interest because, without the deduction, my taxable income had increased.

Needless to say, I've been more proactive about learning what my responsibilities are since then."

-- Brent Shreve, 28, chief marketing officer, West Lafayette, Ind.

Tax Offender Tale #6: 'I Assumed Income From My Side Gigs Was Tax-Free'
"A number of years ago, I started picking up consulting work in addition to my full-time job as a physician. The extra money was great--I used it to travel, invest more and have fun.

But I made a huge mistake in failing to research what I needed to know about having more than one income stream -- and didn't think that I had to pay taxes on the money earned from my side jobs.

I actually remember thinking how great it was that my clients paid me 'in full,' instead of taking out taxes!

By the time my accountant informed me that I should have been estimating my taxes and paying quarterly, it was too late. I'd accrued fines, as well as a tax bill of $7,500.

You can bet I'll never make this mistake again. I talked to my accountant at the beginning of the following year about how much money I needed to put aside for taxes -- and made sure to do just that."

-- Pat F. Bass III, 45, internist and pediatrician, Shreveport, La.

*Names have been changed.

This post originally appeared on LearnVest.

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