When adding Recent Activity as a sticky block, Sticky Blocks break and will no longer allow the administrator to add or remove sticky blocks. The error:
Fatal error: Call to undefined function: print_recent_activity() in /blocks/recent_activity/block_recent_activity.php on line 27
is displayed and will not allow any changes.
as line 7 of /admin/stickyblocks.php appears to fix this issue.
This problem may affect more than just Sticky Blocks.
MDL-6380 (open) has been filed regarding this issue with quizzes, and MDL-5931 (reopened) has been filed regarding this issue with the database module.