Scaling an image down (whether grayscale or colour) using Lancsoz sometimes produces vertical black lines, 2 or more pixels wide, down the entire height of the image.  I've seen both a single black line and two black lines.  Scaling with cubic interpolation makes it go away, as does saving the image, quitting gimp and trying again.

If I select part of the image containing the black line and paste it, I see the black line in the new image.

If I use the eyedropper, it reports it as there.

I don't yet have a reliable way to reproduce this, but it seems to be new in the past day or two.