I put a fence up between mine and my neighbours house a couple of years ago. They had a kind of porch on the back of their house covering the whole width of the house. Technically the fence would be in their garden and they'd lose a few inches of ground, but it made sense to come off the end of this porch contraption. They agreed. Fence went up.

Fast forward two years. They take down the porch contraption and put up their own fence, inside the one I put up, reducing their garden size further. My son goes out to mow the lawn. Goes up the first fence as you would. They lean over the fence and say you're in our garden

He tells them to stop being such daft pricks. They haven't spoken to me or us since. I'm crushed.