I was walking in the nether and I encountered what I can only describe as a phantom block. I tried to step down but I stayed on the same level and vibrated, walking slowly across thin air until I eventually dropped, see screen recording below. 

It stopped when I placed blocks in the "phantom block" then dug them out.

https://watch.screencastify.com/v/mC58CgWUj3l7gIqVR7G0