What is a modern day example of an issue that involves federal vs. state power?

Feds intermittently raided mj crops in Ca. It was legal in Ca. but not federally.

Federal courts interfering in state election laws

Entering the US without inspection is against federal law but certain "sanctuary" states encourage it and protect those that do it. Likewise marijuana is illegal federally but has been legalized in several states. In fact Oregon just decriminalized Schedule 1 narcotics. Like that state needs more drug addled zombies running around. 

Here in MI the state has wanted to allow wolf hunting in areas where there are too many and cause problems. However, the federal endangered species law didnt allow any culls at all, not even in counties overun with wolves.