I noticed an area like that in Tucson, AZ.
The President of TopoZone was nice enough to explain why this occurs.
The full discussion, with pictures of the alignment problem, is
located here.====================================
From Ed McNierney:
I'm not certain about this location, but this is likely a datum error. Many of the DRGs distributed by the USGS were labeled with the wrong datum; GeoTIFFs labeled as NAD27 were actually NAD83 and vice versa. For TopoZone we spent a huge amount of time finding and fixing these errors. If they're not corrected they produce a similar effect to what you show here.
A DRG is an image of the entire quad sheet, with the map collar (margin) included. You need to clip off that collar to mosaic the DRGs together. You know that a given quad covers a rectangular area from, say, 30.125 to 30.25 latitude, and -80.5 to -80.375 longitude. You clip that rectangle, containing only the map image, out from the entire DRG. By doing so you get exactly the map image, and only the map image. This process is considerably complicated by the fact that the DRG is in the UTM projection and those boundaries are not a rectangle!
But you need to know what datum you're talking about, and there are two datums involved. There's the datum used for the original printed map sheet, and the datum used for the digital DRG image file; they are not necessarily the same. If you have a DRG and you think it's a NAD83 image but it's really georeferenced to NAD27, your clipping rectangle will be off by a few hundred feet or so. When you clip out the map area you won't get exactly the map area inside the margins - you'll miss a bit and include a few hundred feet of white map margin where the map's supposed to be. That looks like what is happening here.
There's another possibility but I don't think it's pertinent here. If the USGS updates a quad/DRG the new quad will be produced in NAD83. If it is adjacent to an older quad (not updated) using NAD27, it is possible that there will be a gap in between the maps and that's NOT an error - there's no map there! If the boundary between two quads is 30 degrees North latitude, and one map ends at 30.0000 (NAD83) and the other starts at 30.0000 (NAD27), those two lines are not in the same place and there will be empty, unmapped space, in between.
