When rising in creative mode, player can easily fly through 1 block thick floor.

To replicate:
1: Create a flat floor
2: Fly under it and hold space
3: After no longer rising (reached platform) press shift for a short time, not releasing the space bar.

I believe it is becouse the player gets smaller when sneaking, and the head goes into the block allowing the player to go through it.

Note: In survival player can take suffocation damage, but will always fall back down