i was straight mining in nether with my efficiency 5 pic and suddenly after breaking a netherrack it became a like a invisible block i was like vibrating in that block like when i press forward in that block i started vibrating and when stepped back it stopped vibrating and when again stepped forward i started vibrating again .I mined its left and right area then also that block was vibrating me and then when i cleared all area around it then that block was gone.