I was embarassed the first time I ran onto some 20lb monofiliment. New bar no big deal. Heard some great stories. Hay twine, blue tarp...
Running onto metal is a big deal. Be safe.[/QUOTE]
A few more.
Cutting into embedded barbed wire along the edge of an old, grown in, pasture. That will get your attention too.
Some people also find it neccesary to drive spikes into trees to discourage loggers.