In Moodle 2.7 and beyond, under certain conditions the side bar block can prevent the course page from displaying, without any option for the user to fix the problem.

To reproduce:

• Create a course in "weekly" format, add a side bar block, and add an activity to the side bar.
• In edit mode, click the "Add section" button on the bottom of the course page twice. (Both the "side bar" section and a new section below it will now be visible.)
• Move the empty section on top of the "side bar" section.
• Click "turn editing off".

From now on, the course page will display an "error writing to database" whenever it is called.

The immediate reason is that block_side_bar tries to correct the numbering of sections, and for that purpose tries to write to the course_sections table when the course page is viewed, which results in a unique index violation.

The actual problem is somewhat deeper I think. (Am developing a patch.)