A careful person can do their own taxes. But there are some traps that can screw up someone who doesn’t pay attention to detail. I help military do their tax returns, and the rules vary from state to state on what military pay is taxed when, and what happens if the spouse is from a different state. Education credits can also screw people up, as can child tax credits and EITC. I’ve seen people who have prepared their taxes on good software, but who were off by over $1000 dollars - in each direction!
File a state tax return claiming a $1500 refund when you can only legally get $200 back is a good way to get in a heap of trouble...and that was on a return that could be done on a 1040 EZ! And yes, it was an error, not fraud.
There are also military guys claiming to be from Florida for tax purposes, yet who own property, vote and have driver licenses in other states - another good way to get in trouble.
I am more at the other end of the spectrum. I know things like how to amortize bond premiums, calculate AMT, and calculate the tax when the income is mostly qualified dividends.