When I submit a reply to a topic where the reply contains an attachment, I receive the following warning:

[phpBB Debug] PHP Notice: in file /includes/db/dbal.php on line 610: Undefined variable: error

In revision 8414, $error was renamed to $this->sql_error_returned within the sql_error function, so I guess

return $error; 

should be changed to

return $this->sql_error_returned; 

as well.