I took over an application that consisted of 4 MSSQL (2005 at the time) databases, hundreds of tables, thousands of stored procedures, maybe a 1/4 of them actual still being used, external links to more than 20 other databases (MSSQL, Oracle and DB2) which all ran from a single "master" stored proc that was kicked off nightly by scheduled job.

The existing documentation consisted of a single word document, about three pages long, describing how to set up the application... on the Sql Server 6 server it had been originally created on two generations ago.