I use the canned software more to avoid computational errors on my part and produce a legible, typed return than for any other reason -- My returns are simple, using the most common forms, so I trust the software to handle that without major errors.

I once used a version of Turbo Tax to prepare my own return and my newly ex-wife's return -- It was a year they were playing around with formats, so the return produced did not resemble the standard forms -- Rather, the form number and line numbers appeared along with the amount entered by the user, and ONLY those numbers appeared, the theory being that the program and the IRS computers would only trade the user-entered fields and would do their own computations from that -- A few calculated fields were printed as markers and to ensure that each form was a match to the rest of the return -- Looked like gibberish to the untrained eye unless one had a copy of the program to run it through.

I did the taxes because in our divorce we split all our stuf, including investments, down the middle -- She had then, with our financial advisor, sold most of the growth stocks and mutual funds and replaced them with income-producing investments, creating a lot of capital gains tax accounting entries and calculations. I already had the copies of statements with the original costs, so I figured it would be easiest for me to do her return and save her the money.

I made the 'mistake' of providing her with only a copy of what was to be sent to the IRS to be scanned and processed, the original so to speak -- I didn't bother to produce or store a paper copy of what the forms would have looked like -- All I retained in my records was the file saved to my computer of the program's results.

Mistake number two was that I sent them to her with a hand-written note to the effect that if she had any earned income in that year, she was going to either have me re-do her return or have a tax preparer re-do it and not to submit the original non-standard form. I didn't make or keep a copy of the note.

Final mistake was that I didn't make a copy of the Turbo Tax program itself, to go with the stored file version of the return.

She had apparently taken a job and received one paycheck in late December of the tax year but lost, discarded or ignored my handwritten note and just mailed the strange return in. Of course, the unreported income eventually popped out at the IRS and they sent something to her about it all -- I was full-timing in my RV and off somewhere with voice mail service just piling up for a few days, so she panicked and went to see a full-bore CPA.

He/she was thoroughly perplexed at first by the printout send to the IRS for scanning and processing, finally recognizing it and reconstructing the entire return for examination and input to the practice's software, along with her previously reported earned income, which took a few billable hours, for which she was subsequently charged. I don't know the details, but from the way she was acting, it's likely that the CPA's one-time learning curve cost more than the original paycheck, much less the tax that was unreported (and probably paid by withholding anyway).

I learned a few things from that incident, for sure!