Look up the legal principle of Unjust Enrichment. I had an oil company deliver heating oil to my house by mistake. When I called them and said they screwed up they told me I had to pay anyway due to the aforementioned principle. I had them come back and pump the oil back out.

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i always mow my neighbors lawn.
in fact just mowed it yesterday


in my last house me and the guy across the street had fun with each other.
when his grass would get really tall (not very often) i would go my initials in his front yard.

my other favorite was to pull his sprinklers across the street to my house......he would do the same.

Delivering oil is something that was easy to fix, pump it back out. When the company fertilized my yard I did NOT want it fertilized, and I joked that I will not make him come vacuum it all up and we can forget it. It caused me to use lots more water and mow my yard twice per week from all the growth, so I was not enriched.

Had we not been home when they mowed, they would have been out of luck. The neighbor has a yearly contract for them to take care of his yard. They would have to come back and mow his yard. The pros always want to get an early start on mowing the yards, since it is their job. Right now it is 28 degrees and I don't want my yard cut short, it is too early. In a few weeks I will do it, but not yet. They came on my property, and cut off grass and modified my yard in a way I did not want. I stopped them because it is the right thing to do. The company likely would have made the crew just eat the time they spent here. The crew foreman would be unpopular with the crew, but that is as far as it can go. I would guess the mowing is $40 or so per week. I have no idea. No one is going to sue for forty bucks.