The journaling feature does a good job of keeping changes to data correct, but when a computer shuts down incorrectly data for a given file or files could be interrupted mid write which would result in corrupt data (unintended data)
Wikipedia Definition
Data corruption refers to errors in computer data that occur during writing, reading, storage, transmission, or processing, which introduce unintended changes to the original data. Computer, transmission, and storage systems use a number of measures to provide end-to-end data integrity, or lack of errors.