DropSync generates its sync previews using a tool called rsync. In theory it should always understand what rsync is telling it, but there are circumstances where it might encounter something unexpected. This is especially likely when talking to a computer where an older version of rsync is installed, or when using custom rsync options. If any errors are encountered during preview generation for a particular synced item then that item will be excluded from the sync unless you explicitly re-enable it. Since many errors are relatively benign DropSync shows them in detail so you can inspect them and decide for yourself
To view a list of lines of rsync output which DropSync didn't understand;