When looking for a sync source, if we consider a node but decide not to use it because it is behind us, we log a message that looks like:

2016-09-14T15:31:11.564+0000 D REPL     [rsBackgroundSync] Cannot select sync source behind our last applied optime: WIN-K6DURTQEFVN:20017

This message would be more useful if it included the optime of the node and the node it is considering syncing from.