I like products google creates. Mostly. Nevertheless, sometimes google does something strange. Something I cannot understand.
There is desktop photo management application Picasa. Google does not update it for long time already, but this is another story.
Many photo management applications allow you to rebuild their database to remove garbage out there and, probably, de-fragment - I do not know details.
Now the question: how to rebuild database in Picasa? Google has an answer. For windows you should uninstall Picasa and install it again!
Again: uninstall and install to rebuild an internal database!