Template loops 

The idea is very simple: a template may not call itself, for example
pagename:template:We'll show them
{{template:We'll show them}}
will give a template loop.

And even this
pagename:template:joke_#1
{{template:joke_#2}}

pagename:template:joke_#2
{{template:joke_1}}
will give a template, although the loop is constructed of more than 1 page.

Understand?