I find that from time to time, usually after doing a ripup of auto route and replacing with manual routing, that ratlines stay and stay and stay like they're etched into the screen.
In some instances it is the track not making contact with a pad or something, but I have had cases where all pads on the net highlight, so they must be connected, yet the ratline STAYS. Why ?
Its not serious in that it has no practical impact, but it's like a false warning that something is wrong.