That the Court gives vertical but no horizontal direct effect to directives is argued to may lead to discrimination.Why?

"[I]t is significant that the CJEU has applied the principle [of direct effect of directives] solely in cases between citizen and Member State, and then only when the directive was for the citizen's benefit and not to their detriment [...] (known as 'vertical direct effect'). The direct effect of directives in relations between citizens themselves ('horizontal direct effect') has not been accepted by the CJEU." Cp. ABC of EU Law, p. 103 f.

Support your explanation with a short example!