I recently migrated my WordPress site to a new server with a new URL. Trying to do a simple text search and replace on a database export to change from myoldsite.org to thenewsite.org will cause problems with serialized data. Because serialized data has an explicit length specified, both the data and the length must be changed.
This site offers some good explanation of serialized data in WordPress:
This site has a great utility that I used successfully to search and replace serialized data: