Have you ever worked on a large project where out of the blue, one day, the client says that editors are not seeing a block of new blog posts in right sidebar on so and so page? It's generally pretty easy to fix this problem. Just go to the configuration form of that block and see what the conditions are for displaying the block. Or it could be a block written by you in a custom module and then you'll need to check hook_block_view() in your module to see where the problem lies. But you are still unhappy because the bug was caught on production.