What do you have in mind? The US?Well, it's based on federalism obviously, and there's constant conflict between states and the federal government over , for example, immigration. Mitt Romney's plans for healthcare is an appeal to states to govern on the subject versus Obamacare, which is federal in concept. Surprising if you can't find more; the Supreme Court gets involved in these battles sometimes. There's always the question of taxation too; the argument over whether the federal government is even entitled to tax nationally is the extreme end of it, but it does descend into squabbles about what is a tax (see healthcare) and the extent to which the federal government has power to tax.
In Europe there's the constant gripe about whether we are heading towards a fully federal system, like the US.