Sync random subset is selected is not available for playlists, which is what I'm syncing and then it always shows the warning. Of course reserve space is also unavailable for playlists.

I cannot reproduce the issue and also by checking the code this should never occurs when
[x] Sync random subset is selected

Couldn't this be just a test error?

I think the warning is correct as it indicates that you won't be able to sync all selected files. But this is one of those warnings that can use the don't show me again treatment and an option to enable/disable it under Tools > Options > Confirmations.

When selecting 'reserve x MB of free space' with a portable device, you still get a warning
"There isn't enough space on the device to synchronize selected files. Would you like to continue?"

Seems the dialogue is a little unnecessary if you've chosen to sync a subset to fill device while leaving space free.

Win 7 x64
Build 1361