We've just had the following error message on our production system which runs on Moodle $release = '2.6.2 (Build: 20140310).
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Debug info: Column 'attemptnumber' cannot be null
INSERT INTO moassign_grades (assignment,userid,timecreated,timemodified,grade,grader,attemptnumber) VALUES(?,?,?,?,?,?,?)
[array (
0 => '45066',
1 => '143186',
2 => 1412843794,
3 => 1412843794,
4 => -1,
5 => '22',
6 => NULL,
)]
Error code: dmlwriteexception

Stack trace:

line 446 of /lib/dml/moodle_database.php: dml_write_exception thrown
line 1080 of /lib/dml/mysqli_native_moodle_database.php: call to moodle_database->query_end()
line 1122 of /lib/dml/mysqli_native_moodle_database.php: call to mysqli_native_moodle_database->insert_record_raw()
line 2684 of /mod/assign/locallib.php: call to mysqli_native_moodle_database->insert_record()
line 588 of /mod/assign/feedback/editpdf/classes/document_services.php: call to assign_base->get_user_grade()
line 66 of /mod/assign/feedback/editpdf/locallib.php: call to assignfeedback_editpdf\document_services::get_feedback_document()
line 225 of /mod/assign/feedback/editpdf/locallib.php: call to assign_feedback_editpdf->get_widget()
line 205 of /mod/assign/feedback/editpdf/locallib.php: call to assign_feedback_editpdf->view()
line 1097 of /mod/assign/gradingtable.php: call to assign_feedback_editpdf->view_summary()
line 1206 of /mod/assign/gradingtable.php: call to assign_grading_table->format_plugin_summary_with_link()
line 729 of /lib/tablelib.php: call to assign_grading_table->other_cols()
line 433 of /mod/assign/gradingtable.php: call to flexible_table->format_row()
line 1330 of /lib/tablelib.php: call to assign_grading_table->format_row()
line 1442 of /lib/tablelib.php: call to table_sql->build_table()
line 1185 of /mod/assign/renderer.php: call to table_sql->out()
line 981 of /mod/assign/renderer.php: call to mod_assign_renderer->flexible_table()
line 221 of /lib/outputrenderers.php: call to mod_assign_renderer->render_assign_grading_table()
line 302 of /local/printpreview/assign.class.php: call to plugin_renderer_base->render()
line 112 of /local/printpreview/assign.class.php: call to assign->view_grading_table()
line 90 of /local/printpreview/assign.class.php: call to assign->view_grading_tab()
line 507 of /mod/assign/locallib.php: call to assign->view_grading_page()
line 53 of /mod/assign/view.php: call to assign_base->view()
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I believe the culprit ist the following code in mod/assign/locallib.php lines 2748 and following:
-------------
if ($create) {
$grade = new stdClass();
$grade->assignment = $this->get_instance()->id;
$grade->userid = $userid;
$grade->timecreated = time();
$grade->timemodified = $grade->timecreated;
$grade->grade = -1;
$grade->grader = $USER->id;
if ($attemptnumber >= 0)

$gid = $DB->insert_record('assign_grades', $grade);
------------
If the attemptnumber is negative then $grade->attemptnumber is not set. Compare to line 2643 where this is done correctly.

I see this bug also in Moodle 2.7.2+ (Build: 20140911). So this might not have been fixed.