Looked around for an apache error log parser until I found this command that will parse your error file ordered by error occurrences:
sed 's^\[.*\]^^g' error.log | sed 's^\, referer: [^\n]*^^g' | sort | uniq -c | sort -n
Output: error occurrences - error text
1 AH00126: Invalid URI in request GET HTTP/1.1
1 PHP Fatal error: Allowed memory size of 536870912 bytes exhausted (tried to allocate 32 bytes) in /var/www/con/libraries/joomla/database/driver/mysql.php on line 425
108 PHP Fatal error: Allowed memory size of 536870912 bytes exhausted (tried to allocate 512 bytes) in /var/www/es/libraries/joomla/database/driver/mysql.php on line 425
156 PHP Notice: Undefined index: select in /var/www/con/cli/form_output.php on line 3
156 PHP Notice: Undefined index: inputfrom_value in /var/www/con/cli/form_output.php on line 2
157 PHP Fatal error: Allowed memory size of 536870912 bytes exhausted (tried to allocate 83 bytes) in /var/www/es/libraries/joomla/database/driver/mysql.php on line 425
202 PHP Fatal error: Allowed memory size of 536870912 bytes exhausted (tried to allocate 32 bytes) in /var/www/es/libraries/joomla/database/driver/mysql.php on line 425
218 PHP Fatal error: Allowed memory size of 536870912 bytes exhausted (tried to allocate 80 bytes) in /var/www/es/libraries/joomla/database/driver/mysql.php on line 425
316 PHP Notice: Undefined index: in /var/www/con/cli/unit.php on line 513
520 PHP Fatal error: Allowed memory size of 536870912 bytes exhausted (tried to allocate 81 bytes) in /var/www/es/libraries/joomla/database/driver/mysql.php on line 425
Gist: https://gist.github.com/marcanuy/a08d5f2d9c19ba621399