From the IRS' point of view, the tax-payer is responsible for paying all of it. You can then turn around and sue to recover any of the fees/fines or penalties incurred as a result of an error made by the accountant, not the actual taxes though.
Seems like simplify or just erradicting the tax code would be more just at this point.
In some jurisdictions, it may help with criminal but not civil liability.
Normally it doesn’t get to that point though. CPAs do not want to be known as the person that misfiled taxes and got a court to rule them negligent. They carry insurance for errors so mostly just end up queitly making it right when there is an error.