When Unlimited Grades are checked, the system does not check to see if the grades exceed the maximum float value of the DB field. When this happens the users get locked out of the gradebook in the course.

Steps:
Check Unlimited Grades in

/admin/settings.php?section=gradessettings
Create manual gradebook entries that are normal scores
Create a Manual Gradebook entry that is 99998
Place it in a new category that uses Sum of Grades
You should be locked out of the gradebook

Error Example:
Debug info: Out of range value for column 'grademax' at row 1
UPDATE mdl_grade_items SET courseid = ?,categoryid = ?,itemname = ?,itemtype = ?,itemmodule = ?,iteminstance = ?,itemnumber = ?,iteminfo = ?,idnumber = ?,calculation = ?,gradetype = ?,grademax = ?,grademin = ?,scaleid = ?,outcomeid = ?,gradepass = ?,multfactor = ?,plusfactor = ?,aggregationcoef = ?,sortorder = ?,display = ?,decimals = ?,locked = ?,locktime = ?,needsupdate = ?,timecreated = ?,timemodified = ?,hidden = ? WHERE id=?
[array (
0 => '222',
1 => NULL,
2 => '',
3 => 'category',
4 => NULL,
5 => '663',
6 => NULL,
7 => '',
8 => '',
9 => NULL,
10 => 1,
11 => 100098,
12 => 0,
13 => NULL,
14 => NULL,
15 => '0.00000',
16 => 1,
17 => 0,
18 => 0,
19 => '8',
20 => '0',
21 => NULL,
22 => '0',
23 => '0',
24 => 1,
25 => '1357141534',
26 => 1357141620,
27 => '0',
28 => '3384',
)]
Error code: dmlwriteexception