Unless you have standing fences based on official surveys, it is pretty easy for people to confuse boundary lines whether they be the land owner or leasor or someone on an adjacent tract. My dad once had a member of the family that owned an adjoining piece of ground try to run him off his own lease. The guy whose land surrounds mine once put a POSTED sign on a tree that was about ten feet on my property and he still has a stand that is sitting about 5 feet onto my property. Boundaries are so poorly marked in some creek bottoms that even people who have owned the ground for 50 years can only guess within about 50 yards or so in terms of where the actual boundary line is. Sometimes the people who think that others are trespassing turn out to be the trespasser.