To reproduce:

1. Setup 2 grade items in a course
2. Set the site settings grade_report_averagesdisplaytype and grade_report_rangesdisplaytype to Inherit
3. Set the course's preferences averagesdisplaytype and rangesdisplaytype to default(inherit)
4. Set one of the grade item's display settings to Letter, the other to Percentage

I get this:

( ! ) Warning: end() [function.end]: Passed variable is not an array or object in D:\VertrigoServ\www\moodle_head\grade\report\grader\lib.php on line 1001
Call Stack
# Time Memory Function Location
1 0.0234 128608 {main}( ) ..(null):0
2 1.8421 16314368 grade_report_grader->get_rangehtml( ) ..(null):162
3 1.8714 16316080 end ( ) ..(null):1001

( ! ) Warning: reset() [function.reset]: Passed variable is not an array or object in D:\VertrigoServ\www\moodle_head\grade\report\grader\lib.php on line 1002
Call Stack
# Time Memory Function Location
1 0.0234 128608 {main}( ) ..(null):0
2 1.8421 16314368 grade_report_grader->get_rangehtml( ) ..(null):162
3 1.8716 16316312 reset ( ) ..(null):1002

Which is caused by a contextid for the item with Letter displaytype which doesn't have a match in the grade_letters table. I'm not sure what's supposed to happen here.