(Note: I believe this is more of an UploadPDF issue than any other Moodle module or core issue, but I'm happy to be corrected and re-report elsewhere.)

When a student attempts to access a specific course (not any course), Moodle execution halts with the following stack trace:

Invalid course module ID
Stack trace:
line 429 of /lib/setuplib.php: moodle_exception thrown
line 112 of /mod/assignment/lib.php: call to print_error()
line 21 of /mod/assignment/type/uploadpdf/assignment.class.php: call to assignment_base->assignment_base()
line 494 of /calendar/lib.php: call to assignment_uploadpdf->assignment_uploadpdf()
line 60 of /blocks/calendar_upcoming/block_calendar_upcoming.php: call to calendar_get_upcoming()
line 280 of /blocks/moodleblock.class.php: call to block_calendar_upcoming->get_content()
line 232 of /blocks/moodleblock.class.php: call to block_base->formatted_contents()
line 924 of /lib/blocklib.php: call to block_base->get_content_for_output()
line 976 of /lib/blocklib.php: call to block_manager->create_block_contents()
line 349 of /lib/blocklib.php: call to block_manager->ensure_content_created()
line 7 of /theme/sdctwo/layout/general.php: call to block_manager->region_has_content()
line 654 of /lib/outputrenderers.php: call to include()
line 612 of /lib/outputrenderers.php: call to core_renderer->render_page_layout()
line ? of unknownfile: call to core_renderer->header()
line 1291 of /lib/setuplib.php: call to call_user_func_array()
line 196 of /course/view.php: call to bootstrap_renderer->__call()
line 196 of /course/view.php: call to bootstrap_renderer->header()

Looking at Apache's error log, the following 3 are shown (sanitised and reduced for clarity):

PHP Notice: Undefined variable: cm in /srv/moodle2/releases/20110920070146/calendar/lib.php on line 494, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP Stack trace:, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 1.

{main}() /srv/moodle2/releases/20110920070146/course/view.php:0, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 2. bootstrap_renderer->header() /srv/moodle2/releases/20110920070146/course/view.php:196, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 3. bootstrap_renderer->__call() /srv/moodle2/releases/20110920070146/lib/setuplib.php:0, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 4. call_user_func_array() /srv/moodle2/releases/20110920070146/lib/setuplib.php:1291, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 5. core_renderer->header() /srv/moodle2/releases/20110920070146/lib/setuplib.php:0, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 6. core_renderer->render_page_layout() /srv/moodle2/releases/20110920070146/lib/outputrenderers.php:612, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 7. include() /srv/moodle2/releases/20110920070146/lib/outputrenderers.php:654, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 8. block_manager->region_has_content() /srv/moodle2/releases/20110920070146/theme/sdctwo/layout/general.php:7, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 9. block_manager->ensure_content_created() /srv/moodle2/releases/20110920070146/lib/blocklib.php:349, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 10. block_manager->create_block_contents() /srv/moodle2/releases/20110920070146/lib/blocklib.php:976, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 11. block_base->get_content_for_output() /srv/moodle2/releases/20110920070146/lib/blocklib.php:924, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 12. block_base->formatted_contents() /srv/moodle2/releases/20110920070146/blocks/moodleblock.class.php:232, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 13. block_calendar_upcoming->get_content() /srv/moodle2/releases/20110920070146/blocks/moodleblock.class.php:280, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 14. calendar_get_upcoming() /srv/moodle2/releases/20110920070146/blocks/calendar_upcoming/block_calendar_upcoming.php:60, referer: http://moodle.domain.com/enrol/index.php?id=322

PHP Notice: Trying to get property of non-object in /srv/moodle2/releases/20110920070146/calendar/lib.php on line 494, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP Stack trace:, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 1. {main}

() /srv/moodle2/releases/20110920070146/course/view.php:0, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 2. bootstrap_renderer->header() /srv/moodle2/releases/20110920070146/course/view.php:196, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 3. bootstrap_renderer->__call() /srv/moodle2/releases/20110920070146/lib/setuplib.php:0, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 4. call_user_func_array() /srv/moodle2/releases/20110920070146/lib/setuplib.php:1291, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 5. core_renderer->header() /srv/moodle2/releases/20110920070146/lib/setuplib.php:0, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 6. core_renderer->render_page_layout() /srv/moodle2/releases/20110920070146/lib/outputrenderers.php:612, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 7. include() /srv/moodle2/releases/20110920070146/lib/outputrenderers.php:654, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 8. block_manager->region_has_content() /srv/moodle2/releases/20110920070146/theme/sdctwo/layout/general.php:7, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 9. block_manager->ensure_content_created() /srv/moodle2/releases/20110920070146/lib/blocklib.php:349, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 10. block_manager->create_block_contents() /srv/moodle2/releases/20110920070146/lib/blocklib.php:976, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 11. block_base->get_content_for_output() /srv/moodle2/releases/20110920070146/lib/blocklib.php:924, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 12. block_base->formatted_contents() /srv/moodle2/releases/20110920070146/blocks/moodleblock.class.php:232, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 13. block_calendar_upcoming->get_content() /srv/moodle2/releases/20110920070146/blocks/moodleblock.class.php:280, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 14. calendar_get_upcoming() /srv/moodle2/releases/20110920070146/blocks/calendar_upcoming/block_calendar_upcoming.php:60, referer: http://moodle.domain.com/enrol/index.php?id=322

PHP Notice: Undefined variable: cm in /srv/moodle2/releases/20110920070146/calendar/lib.php on line 494, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP Stack trace:, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 1.

{main}

() /srv/moodle2/releases/20110920070146/course/view.php:0, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 2. bootstrap_renderer->header() /srv/moodle2/releases/20110920070146/course/view.php:196, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 3. bootstrap_renderer->__call() /srv/moodle2/releases/20110920070146/lib/setuplib.php:0, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 4. call_user_func_array() /srv/moodle2/releases/20110920070146/lib/setuplib.php:1291, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 5. core_renderer->header() /srv/moodle2/releases/20110920070146/lib/setuplib.php:0, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 6. core_renderer->render_page_layout() /srv/moodle2/releases/20110920070146/lib/outputrenderers.php:612, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 7. include() /srv/moodle2/releases/20110920070146/lib/outputrenderers.php:654, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 8. block_manager->region_has_content() /srv/moodle2/releases/20110920070146/theme/sdctwo/layout/general.php:7, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 9. block_manager->ensure_content_created() /srv/moodle2/releases/20110920070146/lib/blocklib.php:349, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 10. block_manager->create_block_contents() /srv/moodle2/releases/20110920070146/lib/blocklib.php:976, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 11. block_base->get_content_for_output() /srv/moodle2/releases/20110920070146/lib/blocklib.php:924, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 12. block_base->formatted_contents() /srv/moodle2/releases/20110920070146/blocks/moodleblock.class.php:232, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 13. block_calendar_upcoming->get_content() /srv/moodle2/releases/20110920070146/blocks/moodleblock.class.php:280, referer: http://moodle.domain.com/enrol/index.php?id=322
PHP 14. calendar_get_upcoming() /srv/moodle2/releases/20110920070146/blocks/calendar_upcoming/block_calendar_upcoming.php:60, referer: http://moodle.domain.com/enrol/index.php?id=322

Default exception handler: Invalid course module ID Debug: \n* line 429 of /lib/setuplib.php: moodle_exception thrown\n* line 112 of /mod/assignment/lib.php: call to print_error()\n* line 21 of /mod/assignment/type/uploadpdf/assignment.class.php: call to assignment_base->assignment_base()\n* line 494 of /calendar/lib.php: call to assignment_uploadpdf->assignment_uploadpdf()\n* line 60 of /blocks/calendar_upcoming/block_calendar_upcoming.php: call to calendar_get_upcoming()\n* line 280 of /blocks/moodleblock.class.php: call to block_calendar_upcoming->get_content()\n* line 232 of /blocks/moodleblock.class.php: call to block_base->formatted_contents()\n* line 924 of /lib/blocklib.php: call to block_base->get_content_for_output()\n* line 976 of /lib/blocklib.php: call to block_manager->create_block_contents()\n* line 349 of /lib/blocklib.php: call to block_manager->ensure_content_created()\n* line 7 of /theme/sdctwo/layout/general.php: call to block_manager->region_has_content()\n* line 654 of /lib/outputrenderers.php: call to include()\n* line 612 of /lib/outputrenderers.php: call to core_renderer->render_page_layout()\n* line ? of unknownfile: call to core_renderer->header()\n* line 1291 of /lib/setuplib.php: call to call_user_func_array()\n* line 196 of /course/view.php: call to bootstrap_renderer->__call()\n* line 196 of /course/view.php: call to bootstrap_renderer->header()\n, referer: http://moodle.domain.com/enrol/index.php?id=322

This seems to affect students only: admins and teachers who have switched their role to student do not receive this error. Also, this is the only course where we receive this error, and other courses running UploadPDF activities appear to be unaffected.