If a 401k is rolled over....

[Sorakairi89]

If a 401k was rolled over into a traditional IRA, and then that traditional IRA was rolled over into a Roth IRA (all of these rollovers occurred in the same year).....would a form 8606 need to be filed?

[Opus 17]

Generally no. You will get a 1099R from the 401(k) which you report as a rollover, and a 1099R from the IRA which you will report as a Roth conversion.  Nothing you did created an after-tax basis in the IRA.  However, may need to file form 8606 if you already had an after tax basis in the IRA.  What is TurboTax telling you?

If you had a Roth option (after tax) 401(k), and accidentally rolled it over to a traditional (pretax) IRA, that’s a non-permitted transaction, and I don’t know how you would go about reporting or fixing it.  

[Critter-3]

No.   You will just need to enter both 1099-R forms  and complete the follow up questions in that section to report the conversion.