In the 1960's jetties were built to combat erosion. On what he thought was his own parcel, P then built a boardwalk and dock. Turns out that he was actually building astride the dividing line between his parcel and G's, but eve G thought it was just P's.
Based on longstanding friendship and with no one ever contesting P's ownership - or, to coin a phrase for this case, leaseship - P let G and other neighbors use the dock and boardwalk for years, more years than enough to satisfy the 10-year period applicable to adverse possession.
Potential trouble started in 1984, when G had her parcel surveyed and only then learned that P's structure had encroached five feet onto hers. She showed the survey to P and she deposed, they both "had a good laugh about it". She let P continue to use the property just as before. The actual trouble arrived only after G sold her parcel to Ds (defendants here), who then advised P that he could no longer use the portion of the structure that they deemed theirs. It produced this declaratory action by P, Estate of Becker v. Murtagh, 19 N.Y.3d 75, 945 N.Y.S.2d 196 (April 3, 2012), and P wins it.