I work at a print shop, and my RIP guy frequently runs into this problem -
I will save a file in Photoshop or Illustrator, snaping the edges to the size of the artboard, and when he opens it and adds mirrored bleed, there is a white line that shows up.
(I know a problem might be the artwork says its at the edge of the artboard, but really is a pixel or so off but this time was different)
I made a 2 by 2 grid of artboards (that were touched the edges of the next), and imported a picture that covers all 4 artboards. Theres no way there was white space on an edge of an artboard because the picture expanded past the artboards.
(INFO - Saving from Illustrator as a PDF/X-3, and using Onyx 10 to rip)