When I accessed a non-existed url in my site, I saw this error:
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Notice: Undefined index: in /home/soulther/dev8.learn-greek-online.com/libraries/legacy/error/error.php on line 339
JError::raise -> Static method JError::handle does not exist. Contact a developer to debug
Error was
Not found
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This happened with a combination of sh404sef and redj. When I dissable sh404sef OR if I disable redj, the problem is gone and the correct 404 error page does appear.

First I contacted the sh404sef developer. He investigated and gave this answer:
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I had taken a backup of your site, and debugged to find out. The bug is in the REDJ extension, which incorrectly assumes that all exception will have a "level" value.

Please take the matter to RedJ developers, the bug is in the file /plugins/redj/redj.php at line 653.
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I used the latest free redj version.