The thin bright line was always on either the top or bottom...not both at the same time. Now that I think about it, I can see that it is the vertical misalignment. With Photomatix merge, you can elect to crop the misalignment off during the merge. CS6 does not have that option; you get the entire 32-bit merged file to deal with after merge.
I think I prefer the manual crop. Seeing the thin line tells me I am still not totally stable with my tripod/mount combo.
With the D300, I could get away with "not totally stable." I guess with the D800 I have to redefine stable when shooting the HDR frames.