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Workaround: use double pipes, as in {{#tag:references||group=x}} 

The 'Cite error references invalid input' error check was removed from cite.php around April 2013 so the error display no longer exists. {{#tag:references|group=x}} still fails but does not display.

The issue here is in how #tag parses <references>. The content *must* be defined first, then any parameters— empty content is acceptable.

(In reply to Gadget850 from comment #2)
> The 'Cite error references invalid input' error check was removed from
> cite.php around April 2013 so the error display no longer exists.
> {{#tag:references|group=x}} still fails but does not display.
> 
> The issue here is in how #tag parses <references>. The content *must* be
> defined first, then any parameters— empty content is acceptable.

That should be:

{{#tag:references|group=x}} still fails but triggers the 'Cite error group refs without references' message.