I came across something unusual making an end grain board today. These strange wavy lines that appear only at the top and bottom of the board, and look identical on the backside. It was fine coming out of the drum sander, and it was only after round one of sanding I noticed it. At first glance it looks like a sanding issue, but no amount of re-sanding would correct this. And the rest of the board is fine. At a closer look, these lines are oddly symmetrical with each other, front side and back, so seems unusual to only have a sanding issue like this.