I saw a bunch of odd behavior. Like the odd black line disappears if I have a header with colspan=2. Which tells me the black line is some kind of background or border. I then
unsuccessfully to reproduce it on jfiddle. No black line.
Then I stripped code out of my example until I had a self contained example. (and no the CSS isn't embedded in the file for real. And external file is something I stripped out to make sure there weren't caching problems.) The following code has a black line. But if I change the link from "Search Archives and Recent" to "Search", the black line goes away.
I'm at a loss for why that would matter. And as to why it only happens in two columns. And how to fix it.