Adobe has a announced (see here) affecting Adobe Lightroom 3.4 (latest version up to nw) and Photoshop plugin Camera Raw 6.4 (latest one too).

Basically, when saving metadata to JPEG files, for specific condition (very complex metadata) and certain cameras, JPG file corruption can occur (so that the JPG might become unreadable).

A fast fix should be on the way but up to now just on a HP Photosmart camera the bug has been confirmed.