If I enter $23,000 into the "Elective Deferrals" box for my Individual 401k, TurboTax fills out its "Keogh, SEP and SIMPLE Contribution Worksheet" as if I had deferred $23,000 to the Individual 401k. This is incorrect, since I did not defer $23,00 to my Individual 401k; I deferred $10,000 to my Individual 401k, and $13,000 to another 401k I had through an employer. This results in an incorrect calculations in Part III, lines 10-21 of the worksheet, with an incorrect "maximum deductible contribution for MP, PS, SEP & Individual 401(k) Plans" (line 21) of the worksheet. If I fill out the worksheet from Pub 560 with my actual numbers, I get a different (higher) result for line 21.
I still think this is a bug in TurboTax. Its algorithm for maximizing contributions for Individual 401ks does not seem to be correctly accounting for additional deferrals that were made to another 401k. You can't just add those other contributions into Part III, line 9, because that distorts the calculations that are specific to the Individual 401k, and you can't force line 9 to be $23,000 for the same reason.
