Courses in hidden categories are showing in search results and causing an error when clicked on.
1) Create category and hide it
2) Create course in that category
3) Enrol a student (or any role that can't see hidden categories/courses)
4) Loginas that student
5) Use course/search.php to search for that course
6) In the results (that will show that course), click on the course
7) Unknown category error is thrown - see screenshot

It's easy enough to hide the courses to hide them from the search results, but I would say courses within hidden categories should only be visible if the category is.