What'll often happen is a well-meaning person will come by, often using an editor that doesn't differentiate between types of bicycling, and change the route to be tagged highway=cycleway. This is the tag for things like paved bike paths, such as urban cycling infrastructure.
The result is now these mountain bike trails, narrow/rocky/tight/steep stuff, shows up in tools that offer bike commuters directions. Just... not great.
So many of us keep an eye on the stuff in our area to see that the tags stay as they should.
I don't know much about land-owners doing vandalism, often people try to communicate, sometimes this works out okay.
I don't know anything about mapping public Wifi access, like in shopping centres.
What I meant is that already some people feel like their city and a 30km radius around it is "their area" and they want to control what and how things get mapped there. It can lead to heated and emotional discussions, we are people after all :) If a big company with lots of human resources would do this, there is hardly any fighting back.
One thing that happened when Pokemon Go got popular is that people (users) were conducting vandalism, in the sense that many pokemon figures were added to the main map of OSM. People have to work really hard to remove these, and they might burn out on this. There is already a lot of vandalism, like people adding their friends drinking shed, or whatever they think of as funny.
Another thing that already happens with volunteers, is people thinking their working area is "their area", while no such concept exists. This can already give a lot of friction among volunteers. I can imagine if billions in money are at stake, big companies with a massive crowd of emplyees might want to really grab those working areas and drown out volunteers. If that gets a big problem, volunteers that were active for years might drop out.