Sometimes when Rek'Sai uses E (tunnel) the tunnel will appear, and Rek'Sai will do the animation of tunneling, but will remain in place. I have experienced this bug many times, sometimes in combat, and sometimes not. I have linked an example form a game today that clearly shows the problem.
I am able to re-crate it in practice tool with some level of accuracy. It will usually only happen when the tunnel is being extended by terrain.
Side note: sometimes when you get a really long tunnel over terrain, she travels through it oddly slow, like half the speed of the normal tunnel travel. Not sure if this is intentional.