I was directed to a thread on feedback.photoshop.com, in which Becky Sowada, an Adobe employee, had written:
There is a bug in which Process Version 2003 photos (only) in catalogs that have been upgraded to Lightroom 5.0 will incorrectly get a "Post Crop Vignette: Paint Overlay" History State applied when further processed in the Develop Module. This in-and-of-itself is innocuous, however, for photos in this condition, selecting a subsequent History State can reset existing settings that follow that state in the History panel. This reset cannot be undone, effectively deleting any settings that have been applied after the selected History State.
We are working on a fix for next dot release of Lightroom. In the meantime, a back-up copy of your previous Lightroom catalog is always recommended and will insure that all settings are recoverable for those who experience this issue.
So, we wait. And we don't edit PV2003 photos (even those which have since been upgraded to PV2010) in the meantime.
PS: watsonm, you have the wrong end of the stick: this is a bug in LR5 and nothing to do with browsers.
Glad that you reported the issue and got a reply. I noticed that and started to worry. history…