Help:Cite errors/Cite error references missing group

Overview
References may be defined within the reference list:

List-defined references may be within groups:

Issues and resolution
A list-defined reference uses a group name that is not used within any  tags. All list-defined references must be used or commented out.

The message also appears if you mistakenly embed sfn or sfnp (rather than harv, harvnb, or harvp) inside  :

This message does not show in preview mode.

Bugs

 * The first defined reference that is not invoked will show an incorrect error message.
 * Define a named ref in without using this named ref results into a wrong error message