When a student's post is rated with a value of 0, the student sees the grade display under their post as:
Maximum rating: (1)

When that same post is rated with a non-zero value, say 4, the student sees the grade display under their post as:
Maximum rating: 4 (1)

That is, when the value is 0, the rating display seems to use a space instead of the actual number.

This currently exists in Moodle 2.3.2 and can be easily reproduced on demo.moodle.org as follows, in the CF101 demo course:

1) Log in as teacher to the CF101 demo course
2) Create a forum with default settings except for:
2a) Change "Aggregate type" to "Maximum rating"
2b) Change Scale dropdown to "5"
3) Log in as student and add a post to the forum
4) Log in as teacher and rate the student's forum post with a 0
5) Log in as student and see the rating.

Under the post, the rating text display should say:
Maximum rating: 0 (1)

Instead, as of 10/26/12 on Moodle 2.3.2, it says:
Maximum rating: (1)

See attached screenshot for the bug.