For a day, or two, I had a thin border appearing around each active element on my Mac OS X, despite the fact that it was not effecting anything negatively, it was extremely annoying for two reasons:
1) it is visually disturbing
2) I was not sure if the system was corrupted, or worse - infiltrated by some spyware
After some research I have learned that I turned ON the feature for visually impaired by executing a shortcut Command-F5.