European citizens can travel in the EU without even noticing that they are crossing borders most of the time. I once got lost on my bike and accidentally ended up in France for example.
And if you're unpopular enough, they can also abuse arrest warrants for political persecution from another country. Case in point, captain Tommy Olsen from Norway, who Greek authorities accused of "human trafficking" for helping boat refugees.
They've deliberately made it very easy to request arrests across borders, so it isn't all rosy. One of the reasons it really sucks for everyone when an EU country starts slipping towards fascism.