Client: "We need an email sent to so-and-so back in June 2012. We can't find it. Can you?"
On the client's Exchange 2010 SP1 server, I assigned myself the "Discovery Management" role under https://server/ecp/?p=AdminRoleGroups. When I created a search at https://server/ecp/?exsvurl=1&p=MailboxSearches, it threw an error shown in the screenshot.
This was solved by following these instructions on a TechNet blog to fix an issue peculiar to Exchange 2010 SP1, and then rebuild the index.
Also learned about some built-in troubleshooting scripts. This Technet article talks about the full-text index catalog. This Technet article is all about "Understanding Multi-Mailbox Search". This article says that the built-in Exchange search is great for finding content, but not so good for legal searches because the data is easily contaminated.