Its a very common glitch and an annoying one happens frequently in Custom survival (mostly void worlds). Its a problem where you shift (crouch) to the edge of a block, then you start placing half slabs to branch out. The half slabs would occasionally go to the another half of the block. For example, I was playing skyblock survival, and I want to expand my island with limited resources. I craft my precious cobbles into slabs and start to "shift to the edge and branch out". I was placing the half slabs on the upper half of the whole block. However, it glitched and placed one on the lower half of the whole block. Then my player fell down that very little distance and the game thought the block was not properly placed. The block disappeared and I fell through it.