Anyone ever run into this glitch?

Every so often, I'll export a PDF from InDesign and preview it in...well... Preview, and some of the links will display a non-printing hairline stroke that doesn't increase in size when you zoom in.

I know that the stroke doesn't print and it's not a problem, but I'm curious as to what type of placed file or other detail causes this to happen. I've never been able to figure it out!