I've noticed a bug where randomly Repelling blast(the knock back skill for Eldritch Blast) will drop off your bar after levelling, or sometimes when resting at camp. If you see it fall off your opportunity bar just save game, quit to menu and reload the save. When the game boots up repelling blast will be back.
So far this workaround has worked 100% of the time, but this definitely shouldn't be happening so I wanted to get it out there.
Anyone else had this issue?