This is how it works in Canada. I would expect that it is roughly the same elsewhere.
First of all, the Tax Software (online included) won't let you make huge errors, or offer a warning if you are trying to use a questionable deduction.
The Tax authority will your return through a very basic audit (one that they do to every return) that checks certain things belong in certain places, based on other information in the return. For example they will check your math, and correct mathematical errors, or check that some value that should have been carried over actually was.
After that, if you aren't claiming certain deductions, you probably have zero to worry about. The areas that will trigger further scrutiny are ones where tax fraud or under-reporting is the most common. Even then, if everything seems fine, that's where it ends.
This part could be different (although if it's legal, I'm sure it happens elsewhere) ... Canada Customs & Revenue Agency (CCRA) will, every year, target certain industries or occupations. One year they may scrutinize Construction workers, another Restaurant workers, or maybe people who were on company-sponsored "conventions" at the Golf Resort, or Sales people who make deductions of auto expenses based on personal / business use ratios.
If you have been "getting away" with certain practices in the past, they might catch you this year. And if they do, they may then go back into past years (limited by law as to how far they can go back).
Once a return is flagged for manual (human) examination, that is one more step where you could get a "pass". There may not necessarily be an audit (yet), the examiner might decide all is fine and that's where it ends.
The remaining returns are slated for audit, or a selection of those (say, every tenth return, or whatever they have the personnel for) are slated for audit.
Being audited isn't the end of the world unless you are really doing something that is clearly illegal and with intent (that also could be different in the US or elsewhere).
I've won one and lost one major dis-agreement, that's over 45 working years, and my tax situations have not always been simple so I am in the group where auditing is more common; there was interest to pay on the one I lost, but nobody was talking about hauling me before a judge or anything like that. Just, no, we're not allowing this, here's your bill.
The bill is another story ... if you think you can avoid paying the tax authorities, well, they make Credit Card default collection agencies seem like timid sheep. You will pay them or they will get the money from you somehow, probably in a way you won't like (like taking your house). So ignore everyone else if you have to and pay the man. That bill was $19,000 and it took me two years to pay it, but pay it I did.
I started doing returns as a way to make extra money going to University (my parents had no money; I worked all through the regular semesters about 20 hours a week painting and drywalling, and all through summer). I don't advertise or promote it to anyone, but I still have a handful of clients that have been with me for decades, and I do their stuff mostly as a favour and at a low (competitively speaking) price.
I have never had one of them be audited or have a major deduction dis-allowed, and I am aggressive and thorough with finding and using deductions clients are eligible for. Same as how I do my own. I mention this because I did at one time take the training course and worked one spring for H&R Block. Companies like that are the opposite ... they claim they seek out every deduction but won't touch anything that might require interpretation or a ruling.
The reason is they advance money on your return, and if a deduction is dis-allowed, they are out of pocket. So they play it safe, no matter what the ads say. They get away with it because nobody ever takes a completed return and all the paperwork to a different accountant and pay to have them essentially do it again. So there is no second opinion, so to speak.
