Roles and permissions

I'll try to explan my problem, as my English isn't very good.

I have teachers assigned to course AB in category 1, but they are listed as teachers for 15 other courses in completely different category (let's say category 34).
This teachers have no other role, hidden nor visible. They aren't globally assigned teachers, and not assigned in that "category 34".

There was one more teacher with same problem, but that teacher I could find in database, in role_assignments table (for only one course), and after deletion of mysteriously assigned role everything was OK. But, for this two other teachers in same database table only one entry exists - just for the course 1 where they should be.

Even more, only teachers, non-editing teachers and admins can see this problem, students don't see "bad" teachers...
As this problem is somehow connected to hidden assignments to course, but I just don't know where problem might be.

What could go wrong? I need problem solving or any kind of hint as this is major problem for me.

Regards,
Vedran Mušica