The Adobe Lightroom team has found a bug in the latest versions of Lightroom and Camera RAW. Versions 3.4 and 6.4 respectively have a problem that occurs with saving metadata to JPEG images that already have unusually large blocks of private camera data. In these instances, the software will cause severe levels of corruption to the images. The team report that this is a rare occurrence, but advise that people working with large numbers of JPEG files should wait for an updated release of Lightroom and Camera RAW as a precautionary measure.
Full details of the issue can be found on the Adobe Lightroom Journal. The bugs should be fixed by update 3.4.1 for Lightroom and 6.4.1 for Camera RAW, both of which are due for release on 27 May.