...then you may have run the newer "Pack UV" operation instead of "Pack Islands" at some point while working with your UVs. While Pack UV does a better job of preserving already stacked UVs while packing, it can misfire on some UVs, resulting in them shattering into individual faces, but still looking like one UV island from a distance . For me, it was one half of the ramp's islands that succumbed to this. You can check for this by using UV sync selection and Face mode:
Press "L" over one or more of the offending broken-off faces in your UV editor and they will highlight in the 3D view. Now you know which mesh they're coming from.
The solution is to re-unwrap that mesh, and then the broken islands will be joined again. Or, if you just ran Pack UV for the first time, then undo and run Pack Islands instead. :-)
