If you are flying directly under a block in creative mode, then press the shift buttun, while pressing the button to fly upwards and then release the sneak button again while keeping the fly upwards button pressd, you move upwards into the block on top of your head.

This seems to hapen because you are smaller if you are sneaking and when you stop sneaking you keep your hight, so a small part of your head moves into the block. If you are in the block you can easily get throught the block.

Edit: seems to be happening since the change of the hitbox in 15w42a