In Lightroom 5.7.1 (OSX 10.10), Lightroom is showing a weird, corrupted version of many JPG (not raw) files in my library:
Opening or previewing the image directly (Quicklook or any other app) shows that the image is just fine:
This is happening only to some folders (but many of them), and is progressive - as I open an old folder, it will re-render the original previews and change from the original, good preview, to the corrupted version.
Selecting 'develop' still shows the corrupted image, too.
Discarding the 1:1 previews has no effect.