Help:Cite errors/Cite error references missing key

Overview
A footnote used multiple times may be defined with a name:
 * content

To invoke a named footnote:

References may also be defined within the reference list:

List-defined references may be invoked by the standard markup or by use of r.

Issues and resolution
All references defined in the reference list must be invoked in prior content. Unused references must be removed or commented out.

Including more than one nested reference in will cause a Cite error.

Misusing a template that was never intended as a citation template and placing it in tags may cause this issue.

A template that creates tags; known instances:
 * Certification Cite Ref
 * sfn and variants
 * Single chart

A template that includes table markup; known instances:
 * Certification Table Entry

Examples
Ex. 1) There are three references defined in the list, but none are invoked in the content: {{pre| Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur.

Bugs

 * Multiple problems with refs inside  blocks.
 * The first defined reference that is not invoked will show an incorrect error message.
 * Nested refs fail inside references block
 * Define a named ref in without using this named ref results into a wrong error message