Indestructible Rek'sai tunnel

Rek'sai can create tunnels that cannot be destroyed, do not count toward her tunnel cap and do not time out. Otherwise, the tunnels are fully functional (Rek'sai can dash through them and can ult to them). I saw this while spectating a friend's game (http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/1804922564/208320628?tab=overview ), where Rek'sai had an indestructible tunnel going from bot lane river bush to near red team's turret. I do not know the conditions for reproducing the bug. It did appear that the tunnel entrances were partly inside a wall.