Line 20 on NY return is not adding out-of-state interest income on exempt bonds. I accurately entered part as "multiple states" on my federal return and part as NY.

How far off? 

1)  Don't forget that 1099-DIV ( box 11 ) and 1099-INT ( box 8 )  forms have NY & non-NY breakouts to add together.

2) Can also happen if you entered some K-1 forms with exempt-interest on them and forgot to include those breakouts in your totals. 

3)  IF you entered some accrued interest as part of a 1099-INT , that can warp the numbers if boxes 1, 3, & 8 are all on the same 1099-INT  (there's a fix for that.  and it doesn't apply for NY vs Multiple state breakout's for box 11 on a 1099-DIV )

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Later: 

4)  values in box 13 of a 1099-INT can be mis-applied if you do the state breakdown for the box 8 amounts, leading to warped totals  (A fix for that too)

5)  Oh yeah, don't forget your 1099-OID's too....if you have any.  Numbers there can add into