The official Android 4.3 build has been available for a while now, but my Galaxy S3 would never upgrade. Finally, I found several tutorials that showed me how. The most helpful was this one. I did not do a factory reset of my device because I did not want to re-install all my applications. But I followed all the other instructions and I successfully upgraded to Android 4.3.
After the upgrade process completed, I had several applications give me a “security notice”. Sometimes it would just tell me unauthorized access has been blocked and sometimes it would tell me which app it blocked. You will know which applications it does not like because you will get the “Unfortunately … has stopped” message. For me, after 2 restarts, these apps were removed.
The solution to fix the problem is to simply re-install the application. If it was not removed, uninstall yourself. After I did all that, everything worked fine.