If 1099-R shows an amount in box 2a but it is not the correct taxable distribution amount, and box 2b is checked, can I adjust Box 2a or will I be flagged by the IRS?

If you believe the amount in box 2a of the Form 1099-R is incorrect then you will have to contact the plan administrator for a corrected 1099-R.

1. What is the code in box 7 of the Form 1099-R provided by the payer?
2. Is the IRA/SEP/SIMPLE box marked?
3. Which of the two box 2b boxes is marked?

Unless this is a regular distribution from a traditional IRA, if box 2b Taxable amount not determined is marked, box 2a should be blank.

If this is a distribution from a traditional IRA reported with code 1, 2 or 7 in box 7 and with the IRA/SEP/SIMPLE box marked, the amount in box 2a is required to be the same as the amount in box 1.  Then, if you have basis in nondeductible traditional IRA contributions, the taxable amount is calculated on Form 8606.  Be sure to click the Continue button on the Your 1099-R Entries page and answer the questions that follow so that TurboTax can properly prepare Form 8606.