As mumkin said, there isn't really a problem with a commercial or private campground, since there you always have recourse to management to resolve the issue if someone is in your site. For state and local government, it's become a lot less of a problem because almost all the parks have gone to online reservations. It hasn't happened to me, but I've seen a case where someone was in someone else's spot. One person has an email from the online system that has the site number and date, so it's pretty easy to resolve. That said, I try to arrive before dark. That helps in backing in, setting up, and resolving potential issues. In the case I saw recently, the person with the reservations said they were leaving early the next morning, so they just arranged a site-sharing agreement. By the way, the reason for the dispute was that the campground hosts had not done a good job of marking which sites were first-come-first-serve and which were reserved, plus it didn't help that the campground used a very non-intuitive system of designating which sites were which.
Anyway, these days, if you have a reservation you should be good. The trickier part is if you don't have a reservation, you need to do due diligence to avoid setting up in a spot someone else has reserved but hasn't arrived yet.