I discovered that this query returns 74 results
MariaDB [dev_civicrm]> SELECT COUNT(*) FROM civicrm_line_item WHERE financial_type_id IS NULL;
+----------+
| COUNT(*) |
+----------+
| 74 |
+----------+
The one I investigated was for 12.93 in Dec 2006 for contact 289721 and is not visible in the UI - I am pretty sure it's because of the missing financial type. The most recent is 2014-08-21 14:07:53 ($7.71 against contact 6724808) - That one IS visible in the UI so perhaps the scope is less but I should figure out & tidy up