Not sure what you mean about it being impossible for there to be a cycle. It’s fairly normal for two peers to open channels to each other so they can do net settlement thus creating a trivial cycle. There’s one funny phenomenon which can happen though: Since funds might have a tendency to flow in one direction over time, nodes might charge a fee for funds travelling in that direction and give a bonus to funds travelling in the opposite direction. It’s possible that an arbitrage opportunity could show up where someone could make tokens just by cycling their funds through the system. That wouldn’t exactly be a problem, more like the system paying someone to figure out their routing issues.