As part of my importing images procedure I have the freestanding dng converter (220.127.116.119) convert all my cr2 files to dng because, among other things, dng files are smaller than the cr2 files they derive from. Having downloaded and converted a batch of 2000 or so images from a recent holiday, LR4.1 complained that one of the images had suffered damage, which manifested as a small square area with multi-colored horizontal lines and blotches. The natural assumption is that the original cr2 was corrupt, but it wasn't. Maybe the converter had slipped up because it had a heavy load and I was doing other stuff on the computer while downloading. So I did the conversion again; and the selfsame damage occurred again, exactly as before. This would point to a bug in the converter?
How are you determining that there is no damage to the orginal file? Likely the damage is there, but the thumbnail you are looking at is the embedded JPEG that was generated by the camera and has little to do with the raw data that the DNG converter will use to create a new copy. Have you tried importing the original CR2 file into Lightroom and seeing whether you get the same reaction? If it works, try using LR to do the DNG conversion.