Screwups can happen at any level. And I think too many people have in mind, "I've handed it over to a professional. They've got it, and I don't have to think about it." You sign, too, and you are on the hook for screwups. Even with the big boys.
During my time in Ireland, I benefited from the Special Assignees Relief Program, or SARP. It basically reduced Irish income taxes to match US rates, for a period of time. It did this by taking a percentage of income off your Gross Income amount, which all their taxes are calculated from. Our company used Deloitte for employee tax services. They briefed us on how it would work, and made projections to help us decide to go there, or not. The first year we did taxes, I was following along, for both Irish taxes (which I was totally new to, but read up on) and US taxes. I couldn't get the US tax calculation to line up, and eventually walked Deloitte (to their annoyance, judging by their tone) back to the Irish gross income.
"So, you begin US taxes based on the Irish gross income?"
"Yes."
"And that is after the SARP credit is applied?"
"Yes."
<me, thinking how to put this>
"So, you're telling me the US Federal government recognizes SARP?"
<silence on their part>
Not long after, everyone's US taxes went up a bit. It was bad news in the moment, but it would have been worse news if someone (the top execs) had been audited. Or, when I tell the US Social Security Administration about my time in Ireland, to apply my Irish social pension payments. If those happened, it is the taxpayer on the hook for penalties; Deloitte disclaimed responsibility.
It became a bit of a legend that I figured out an Irish tax provision and had to connect the dots for them.