My recent objectives are to migrate several old websites (circa 2001-2002) to Apache Tomcat from IBM WebSphere to cut costs. At the same time, Microsoft SQL server was in need of an upgrade as well. I've actually enjoyed finding the problems and solutions. Most of the issues were easy to fix (albeit time-consuming in some cases), and the objective has allowed me to know the websites through-and-though. (Unfortunately, documentation or user guides could not be located (most likely because none was written), and test specifications needed to be written with little or no knowledge on the websites.)
The majority of the migration issues amounted to incorrect links (one website used relative links without the slash in front of the path or filename), missing JAR files, data errors, and Java 'out of memory' errors when some functions were executed. (The following command could be executed on the Tomcat installation to increase the memory: set CATALINA_OPTS=-Xms512m -Xmx512m.)
The most important lesson for companies to learn from this is to ensure that their websites/software are documented well, documents are updated, and test specifications are created and updated.