Oracle Database Discussions

Below quoted sentence is from Oracle documentation

"Online redo logs, unlike archived logs, should never be backed up. The chief danger
associated by having backups of online redo logs is that you may accidentally restore
those backups without meaning to, and corrupt your database"

Can any one explain , how the database will be corrupted if we backup online redo logs. I am having a hard time to understand this quote

thanks for your help

shajan