Transparent textures issue

Often when you use transparent textures you will get a black line in game, like the left trees in the below picture.



It`s even possible to see a dark band at the top in hammer, circled in yellow.



Some might say it's caused by the top of the brush being drawn, but in the above image all sides are covered in null.


So whats the problem?

When you align the texture to the top, what seems to be happening is that the top is being blended with the bottom of the texture making a visible dark line.
Luckily it's easy to fix when you know how,  either shorten the brush by a single hammer unit ( in red below) or move the texture up by one