I already have an irrational fear of piano wire strung between posts/trees.
We had some guy here in New Mexico doing exactly that across MTB trails with thin, stainless steel wire between trees across a steep, fast, downhill singletrack. At least one rider hit one of the wires and suffered some minor cuts to his arm–fortunately he was a tall guy and it hit him below his neck.
A hiker spotted him placing other traps and followed him to his car,then reported the description and license number. Since it was on federal land, he ended up in federal court and was fined $2000, 360 hours of trail repair work and 1 year of probation.