When gradebook categories are setup, with nested categories, e.g.
------Unit 1 Graded
------Unit 1 Ungraded
------Unit 1 Test
------Unit 2 Graded
------Unit 2 Ungraded
------Unit 2 Test
The student gardebook reflects this structure. Specifically, the parent categories ('Unit 1' and 'Unit 2') have leading cells that have a rowspan value that means the child categories are visually contained within the parent.
If one of the sub-categories (e.g. 'Unit 1 Ungraded') is hidden by the teacher, the student gradebook becomes malformed. The parent category (in this case 'Unit 1') has a leading cell rowspan value that includes the hidden child category, even though the child cells are not shown. This incorrect rowspan value deforms the remainder of the gradebook table.
This has been discussed in the forums (where there are screenshots showing the issue), where I've also posted a patch...
(This seems are very visible bug, yet I couldn't find it in the Tracker, nor many mentions in the forums - strange!)