In the multichoice question types it seems that negative grades (for example -50%) don't work. If your answer is wrong the result is allways 0.

I searched in the code, and it seems that the problem is in the function grade_responses of the file questiontype.php for multichoice question types.

It seems that this line must be redone to allow negative grades:

$state->raw_grade = min(max((float) $state->raw_grade,

0.0), 1.0) * $question->maxgrad

http://moodle.cvs.sourceforge.net/moodle/moodle/question/type/multichoice/questiontype.php?view=log