I get frequent warnings in my logs when a nonexistent entry is requested via URL. This happens quite often, like this:
A common source of these log messages is web/edit_entry_handler.php, line 363. While this message may be of interest during development, it's just unnecessary noise in production, as this situation is perfectly normal in practice (happens about once per week here).
Unfortunately, I cannot suppress E_USER_WARNING messages in general via configuration, because error_reporting is overwritten in internalconfig.inc.php.