This is a weird one, because I cannot for the life of me track down what element is adding the random lines. Here's a version of an article which works correctly. Note the complete absence of any code elements. Now, I recognize that this version has loads of CSS classes and other weirdness, so take a look at the same article, sans that, but with a code tag and you see weird lines.

I've inspected the document, element by element, and I cannot figure out where those are coming from. It doesn't happen every time I use a code tag, but it happens enough that it's annoying. I have no idea if it's a stray border, or a stray text decoration, or some other weird bug. Happens in both FF and Chrome in my testing.