(1) what is another example of an issue that has become a problem because of differing ideas/laws from the state to the federal level?
(2) what is the current status of this issue today and is it still a problem?

A very modern example of this can be regarding same sex marriages. Although it was a state by state thing, it was declared by the supreme court that banning it was unconstitutional and that all states have to allow it, it became a federation wide thing. The current status of the issue is that all states have to allow it but many are unhappy because of it so they try to avoid doing it, but they ultimately do do it because they have to obey the supreme court.