I work in sports broadcasting for the company that produces the coverage for the World Cup, consequently over the past years I've seen a lot of spider cam work, testing, footage etc... and to me it looks exactly like a spider cam does when it is being quickly moved back to the centre of the pitch.
That motion would occur shortly after a period of action at the goal, where the spider cam would have followed the play and found itself located down over the goalmouth. As the goalkeeper is preparing to take a goal-kick the spider cam would be quickly pulled back into a default position over the centre of the pitch. That is exactly what is happening in this clip...
https://www.youtube.com/watch?v=o9sbIJgmfqc
However, this video is different -
https://www.youtube.com/watch?v=mh8LgRA13Ao - and clearly looks like a man. However, we all know that what we see in this video is physically impossible. Nobody reacts. This clip is taken from a different angle and to me the guy running down the stairs seems to be jerky, like a CGI addition.
My theory, someone has noticed the spider cam on the broadcast footage and mistaken it for a 'phantom'. Someone with access to SuperSport's other camera angles has taken the wider shot and using effects software this second video has been produced adding in the guy running down the stairs before merging with the spider cam.
Although... what I would say is that it is highly unlikely that a Bolivian league match would have a spider cam system installed. This is one of the less high-profile South American leagues, it is not a country that has an upcoming FIFA junior event so would be no reason to be testing such technology. Even English Premier League matches do not have spider cam coverage, the last World Cup only matches at five venues used it.
It can't possibly be a guy running through the seats. It's not a ghost. Spider cam + CGI trickery remains the most logical solution to me. My conviction has been nudged slightly though...
EDIT: The match was actually a Round of 16 Copa Libertadores match, which is the equivalent of the Champions League in South America. This makes the use of spider cam much more plausible.