I am working on a tax return where the taxpayer falls into a 24% tax bracket. This taxpayer completed their tax return on Turbo Tax and is benefiting from a 17.4% rate???

You're probably looking at the highest bracket, TT is probably figuring the average rate.  What is the difference in tax, between the two programs?

FWIW I just stuck that 205K number on a  W2 for MFJ and ProSeries shows me top tax bracket is 24% and effective tax rate is 15.34%  so there must be some other stuff entered in TT to get the rate theyre seeing.

"and is benefiting from a 17.4% rate"

You are comparing two things that are not the same. One is a Component of the other. Any specific item that has its own tax Rate will change the Average from the bracket, as well. How much of the income flows into the next tax bracket will increase the Average, too, because Average is based on the Sum of income. If their income is barely into that next tax bracket, their Average goes down by comparison to an income that hits near the top of the same bracket.

That's why it's called Effective Rate. If we had a Flat Tax system, then your comparison would be valid.