Google "adverse possession"
Working for a council it's something we come across all the time, as we own so much land we can't be constantly checking on it. So for example, if someone's been mowing the grass verge in front of their property for years, they can then claim they own it. Of course, when it's not a grass verge but a flat or larger piece of property that's fallen through the cracks at some stage it can pose a big problem.
In NZ there is still a doctrine of adverse possession but it's not nearly as wide as in the UK.