Microsoft Windows Vista includes a number of folders which are actually links to other folders. In some cases they are recursive links, i.e. they link back to one of their own parent folders. This effectively means the folder contains an infinite number of child folders. SyncBack can detect these infinite loops of folders. When it detects them a non-critical error is recorded in the log file that states which folder (and where the folder points to). For example, the \Users\[username]\AppData\Local\Application Data\ folder links back to \Users\[username]\AppData\Local\
If you are using Windows Vista you can safely ignore these non-critical errors if they refer the to \Users\ folder.
For more information on junction/mount points see this article: