Hot backup question on open source?

[cfigurelli]

Hot backup question on open source?
I have been playing with Zimbra for a couple of weeks now and am enjoying it very much. I have a question about hot backups on the opt/zimbra directory.
Tim Brennan mentioned that bad things could happen if the database changes due to an email floating in while a backup is in process. Did he mean it could destroy your mail server or just give you a bad copy to restore from?
I use webmin file system backup for timed daily backups to a USB external just to take off site if needed and I have a RAID mirror for real disaster recovery. The USB drive is just a last resort. Is the hot backup a threat to my stable system? Some input is greatly appreciated.