Something that catches out a lot of new SSIS developers. The caching mode used for a Lookup Transformation may affect case sensitivity.
The Full Cache option is case sensitive by default. Partial and No-cache options use the Collation setting of the database (or table) to handle case.
Of course, using Partial or No-caching results in the SSIS package issuing a call to the database for each row in the data flow, so use them wisely!