Trump and Mexico - China must be paying very close attention to this dustup as they try to figure out how they're gonna deal with Trump and one imagines how they hope to manipulate him. Diplomats fuss over language because a careless phrase here or there can easily escalate into the ‘hardening’ of certain positions that can then lead to who knows what. So one can legitimately argue Trump insisting on Mexico paying for the wall put Nieto in a very tough position - obviously couldn’t agree without doing himself harm but also I guess neither could he simply say nothing and let it pass. So blame Trump for that. On the other hand, Mexico seemed to be acting in a way that was oblivious to the reality that the ascension of Trump has changed everything - for Nieto to be going around mouthing left wing talking points about immigration and border security to a guy who built his brand on getting tough about those very things was not a great idea - especially when we all know Mexico’s love of open borders has absolutely nothing to do with lofty ideals and everything to do with cynical self interest.
So I’d say that Nieto is the first instance of a world leader being wounded by a failure to recognise that with the ascension of Trump the rules have changed - which is why China must be paying very close attention to this.