To reproduce:

1. Check out a patch before MDL-72092 has landed:

   git checkout 47e880e347ccb2c216d72f803ff6154820695bf0

2. Open the block_instances table.
3. Log in as an admin and go to the Dashboard and then check the block_instances table. You'll see block instances for that admin's user context ID.
4. Log in as another user and go to the Dashboard. Checking on the block_instances, you'll see the records for the user's context ID.
5. Check out the recent version where MDL-74271 has been included.
6. Run the upgrade
7. You'll see three "recentlyaccesseditems" records but their parentcontextid is set to 1 (system). Two of these (the one for the admin and the user) should have the parentcontextid set to the admin's and user's user context ID, respectively.
8. Go to Site administration / Apperance / Default dashboard page.
9. Reset the Dashboard for all users.
10. You'll see the three "recentlyaccesseditems" records on the block_instances. The admin's and the user's block instance records are deleted.
11. As admin, view the Dashboard again and check the block_instances table.
12. You'll see that the new block instance records for the admin's context ID are created. Same will happen for the user. The extra recentlyaccesseditems block instance records are now orphaned.