Yesterday evening when downloading the latest images in my long running Mirror lens test review, I was faced with this as Lightroom corrupted the images after transferring to my MacPro and during the preview build and /or DNG conversion. You can see that the images initially appeared OK but then as the previews built, bizarre lines and colours appeared.
The solution was to delete Lightroom preferences, like this.
Restart Lightroom,
On Mac holding down SHIFT OPTION COMMAND – or SHIFT ALT COMMAND
Then RESET PREFERENCES.
The problem then is that you have to reset all your preferences such as the location of cache, export file format etc. But at least it solved the problem.
Wayne Grundy's PhotoTech Blog 