When I turned 59 1/2, I took an In-Service withdrawal from my 401(k) and rolled 90% of the balance to my Traditional IRA. Later, when I retired, I took the remainder and rolled it over to the same Traditional IRA. I had make after-tax contributions to the 401(k) during the 2001-2003 time frame. During that time, my employer switched the 401(k) from Fidelity to MFS. When I took the In-Service withdrawal and rollover in 2006, MFS credited me with 12K in after-tax contributions, which I reported on Form 8606. When I rolled the final 401(k) balance over in 2009, MFS credited me with another 1K of after-tax contributions which, through oversight, I failed to report.

Now, 5 years later, I am preparing to take RMD's from the IRA. In checking the MFS statements against my pay check stubs, I discovered that MFS failed to credit me with the after-tax contributions I made before the 401(k) was transferred from Fideility to MFS (they were characterized as transfers, rather than contributions). My bad for not checking the after-tax amounts at the time, but I under reported them by about 4K in 2006.

Is there any way, at this late date, to avoid paying tax again on the 5K (4K from 2006 and 1K from 2009) that was either under-reported (2006) or unreported (2009)?

in reading the instructions for Form 8606, it appears there is a $50 penalty for failure to file, but it says nothing about other consequences. Could I just use the correct total in calculating the RMP in 2015 and pay the $50?