@DaveF1006 @Samuel R
True, but it really depends on what was entered in box 2a for the 1099-R on the original tax return, and whether the correct taxable amount transferred to the form 1040.
My yearly 401k distribution (equivalent to a TSP), the 1099-R always has box 2a already filled in with a value $109 lower than box 1, (with box 5 = $109) since I had some after tax contributions to my 401k.
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Checking a test file with a 1099-R, where box 2a was lower by $2222, TTX didn't complain if I put $4444 in box 5...and...the 2,222-lower box 2a amount was transferred to the form 1040, without dealing with the $,444 value at all. So the fact that the original box 5 was 2x higher than the reduction showing in box 2a made no difference, and the taxable amount reported in box 2a remained (at least on my desktop software).
So...in thus OP's situation, when he entered the original 1099-R, was that original's drop form box 1-to-box2a equal to that original box 5a value, or was the drop actually half his original box 5a value?
Or...was box 2a empty on the original....in which case, perhaps the entire box 5 value was used to reduce the box 1 value...then an amended tax return would be needed.
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*
