1. Start with a course that has some stuff in it. 2 categories in my case, no extra settings different to the defaults.
2. go to the edit categories and items page
3. click on the uppermost edit icon in the cateogry table, next to the course name
4. set the display type to 'Real (letter)' and click save
5. get this error:

Out of range value for column 'decimals' at row 1

UPDATE mdl_grade_items SET courseid = '216',categoryid = NULL,itemname = '',itemtype = 'course',itemmodule = NULL,iteminstance = '269',itemnumber = NULL,iteminfo = '',idnumber = '',calculation = '=sum(##gi742##,##gi744##)',gradetype = '1',grademax = '100',grademin = '0',scaleid = NULL,outcomeid = NULL,gradepass = '0.00000',multfactor = '1',plusfactor = '0',aggregationcoef = '0',sortorder = '1',display = '13',decimals = '-1',hidden = '0',locked = '0',locktime = '0',needsupdate = '1',timecreated = '1212392853',timemodified = '1241515328' WHERE id = 726

• line 1704 of lib\dmllib.php: call to debugging()
• line 205 of lib\grade\grade_object.php: call to update_record()
• line 277 of lib\grade\grade_item.php: call to grade_object->update()
• line 593 of lib\grade\grade_item.php: call to grade_item->update()
• line 160 of grade\edit\tree\category.php: call to grade_item->set_hidden()

Out of range value for column 'decimals' at row 1

UPDATE mdl_grade_items SET courseid = '216',categoryid = NULL,itemname = '',itemtype = 'course',itemmodule = NULL,iteminstance = '269',itemnumber = NULL,iteminfo = '',idnumber = '',calculation = '=sum(##gi742##,##gi744##)',gradetype = '1',grademax = '100',grademin = '0',scaleid = NULL,outcomeid = NULL,gradepass = '0.00000',multfactor = '1',plusfactor = '0',aggregationcoef = '0',sortorder = '1',display = '13',decimals = '-1',hidden = '0',locked = '0',locktime = '0',needsupdate = '1',timecreated = '1212392853',timemodified = '1241515328' WHERE id = 726

• line 1704 of lib\dmllib.php: call to debugging()
• line 205 of lib\grade\grade_object.php: call to update_record()
• line 277 of lib\grade\grade_item.php: call to grade_object->update()
• line 549 of lib\grade\grade_item.php: call to grade_item->update()
• line 163 of grade\edit\tree\category.php: call to grade_item->set_locktime()

Out of range value for column 'decimals' at row 1

UPDATE mdl_grade_items SET courseid = '216',categoryid = NULL,itemname = '',itemtype = 'course',itemmodule = NULL,iteminstance = '269',itemnumber = NULL,iteminfo = '',idnumber = '',calculation = '=sum(##gi742##,##gi744##)',gradetype = '1',grademax = '100',grademin = '0',scaleid = NULL,outcomeid = NULL,gradepass = '0.00000',multfactor = '1',plusfactor = '0',aggregationcoef = '0',sortorder = '1',display = '13',decimals = '-1',hidden = '0',locked = '0',locktime = '0',needsupdate = '1',timecreated = '1212392853',timemodified = '1241515329' WHERE id = 726

• line 1704 of lib\dmllib.php: call to debugging()
• line 205 of lib\grade\grade_object.php: call to update_record()
• line 277 of lib\grade\grade_item.php: call to grade_object->update()
• line 507 of lib\grade\grade_item.php: call to grade_item->update()
• line 164 of grade\edit\tree\category.php: call to grade_item->set_locked()

Error output, so disabling automatic redirect.