We have been seeing the following issues on our moodle 1.9.12 instance. The problem is that the "action" field of the mdl_log table is limited to 40 chars. This means that the below SQL is failing.

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Insert into log table failed at Thursday 12th of May 2011 12:00:41 PM.
It is possible that your disk is full.

The failed SQL is:

INSERT INTO mdl_log (time, userid, course, ip, module, cmid, action, url, info)
VALUES ('1305165641', '5', '1', '165.228.100.81', 'scorm', '11', 'trk: Clinical Management of the HPA Axis at: 1', 'view.php?id=11', 'cmi.core.score.raw => 0')

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We use postgres for our Moodle instances, this probably doesn't fire a bug with MySQL (which will sometimes just concatenate data to fit smaller data fields)