I don't know what causes the artefacts I talked about earlier. I do know they show up when you resize images on export. They also show up if all you do is crop your image. I have generated an example of this, again using the car image from before. I once exported the image uncropped from Lightroom to sRGB tiff, and I cropped it and again exported it. I took the two files into Photoshop, scaled them both to 200% using nearest neighbor scaling and aligned them to each other using the image align built in. Here is the result again using the mouseover trick. The default shows the image that was not cropped in Lightroom. I also include a substracted image that shows just the difference between the two calculated using apply image. The images can take a while to load after you hover so be patient. The change will be VERY obvious.
Not cropped in Lightroom
Cropped in Lightroom
You see that is pretty bad. You see it is not just an extra halo that gets created, but also a sort of combing effect arises. Again Lightroom did not scale upon output. This was just the result of cropping. You can see these artefacts immediately in the Develop module in Lightroom when you just cropped.
P.S. I am not the first to point out this problem. Many people have done so before me (example link). I hope this will soon get fixed. You need light objects on dark backgrounds to see these effects (or the inverse). You see that even in pretty standard sort of images these problems can show up. If your image is not so sharp to start with this (the crop-induced artefact) will probably not be an issue. This was shot with a very sharp prime lens that outresolves the D300. However the resizing artefacts will show up even for less sharp images.