How to reproduce:
1. Have block above player head
2. Fly up into the block so head is touching the bottom of the block
3. Keep hold of space-bar
4. Tap crouch key whilst still holding space-bar
5. The player will glitch through the block above

This happens in single player and multiplayer.
It only happens with 1x1 blocks, but if you keep hold of space-bar and spam the crouch key then you can glitch through multiple layer of blocks.