Just lucky it was rope and not piano wire. Heard all kinds of horror stories (some probably urban legends) back in my dirt biking days of things like that. Whenever I encountered an unflagged/unmarked cable, chain, etc. on my rides, I always hung as much stray trailside crap on it as I could find to help make it visible. Which reminds me - - roll of surveyors ribbon should go in my camelbak as standard equipment.
I hate stuff like this..I've heard about fishing line being strung up across the valley from me in the Pfalz. On my route I've come across numerous dangerous things set up to hurt people intentionally. Glass embedded into corners, massive piles of rocks at the place where you go against a berm after going down a really steep segment, and many, many branches with some laid out parallel to the trail in corners. The stupid thing is that they are most likely to hurt a beginner or someone not familiar with the trail.