If I have facing pages setup with art that touches the inner page edge but does not go over that page border there is a tiny slice of that art that shows up when exporting a pdf. My client always notes it as an error. Example: left hand page has an image that fill the page and bleeds off the left but only touches the right hand side of the page. I use align to right of page so it is right on the edge. I export to pdf and a teeny tiny sliver of that image shows up on the right hand page. Evident when viewing the pdf as a single page. I have to move the image away from the edge by a 1/4 pt to fix.