I just got some boards back from OSHPark. They are test-jig boards with a couple card-edge connectors for attaching the board-under-test.
I was doing a little continuity testing to make sure they were what I designed and discovered a short between a bunch of pads. I couldn't see the problem with a visual trace, so I went back to the board design in KiCAD and confirmed the nets were not connected. Taking a microscope view of the board, I discovered the problem: I had included the board outline on the copper layers and OSHPark tried to subtract the outline from the copper but missed 1 mil or so on the left side, front. This hairline was the resulting short. Fortunately I can just grind a bit of a bevel on the board and remove the offending copper.
The lesson here is, always look at your gerbers. I would have immediately seen the outline layer and went back to remove it before I sent the gerbers to the fab. Even though OSHPark does a render of what the board will look like—which I counted on to be correct—there is no sense in sending them files that have such obvious bugs.