Lightroom Product Manager Tom Hogarty has reported a Lightroom 3.4 and Camera Raw 6.4 bug that "could cause image corruption which falls within our highest severity category of bugs."
Hogarty recommends waiting for updated releases of the affected software if you work with a significant number of JPEGs and routinely update metadata. The Lightroom 3.4.1 and Camera Raw 6.4.1 updates are expected to be released by Friday, May 27, he said in a Lightrooom Journal post.
But he also pointed out that the issue is so rare it isn't necessary to downgrade to the previous version of Lightroom or Camera Raw. In fact, Adobe is aware of only one camera affected by the issue, the HP PhotoSmart R607.
The issue was discovered by Doug Cooper five days ago, who reported it via the Lightroom Feedback Portal. After restoring images he had noticed were corrupted in his Lightroom catalog, he reopened Lightroom and navigated to the restored images.
"The images I had just restored started updating their thumbnails in LR, and right before my eyes several of them came up corrupted. I looked back in Finder, and sure enough, the files were bad again -- with a current timestamp on them. It appears that LR had updated the files with a new thumbnail or metadata when reading the images, and corrupted them in the process," Cooper wrote.
Hogarty explained that the JPEGs "must have an unusually large block of private camera data included in the file to be susceptible to the bug."