I thought you might like to know the resampling algorithm that caused those ugly "ringing" halo artifacts in older ACR's and Lightroom has been replaced with a better one. The usually appear around the sharp edges of bright objects against a darker gray or lack backgrounds.
If you never encountered those artifacts, take look at http://farm4.static.flickr.com/3032/2870105334_6117efb779_o.png
. Here's what happening if the crop you made happened to fall "in-between pixels", thus needing to resample the cropped are to the closest integer value.
Another example, would be Figure 6-42 in The DAM Book
. Note the artifacts on the window frame edges. In this case, you can see that the image has been downsampled to 6.3 Mpx from the native 12.4 via ACR's menu.
Now these are gone! ACR 5.2 does not have them and the upcoming LR 2.2 is expected to get rid of them too.