When you walk through the structure shown in the attachment, flying is automatically enabled. (Creative Only)

_edit:
The specific structure illustrated isn't necessary. Moving from walking on any slab to floating in water (where the water height is less than 1 block above the slab height and where autojump is enabled) will enable flying when in creative mode.
Only happens when walking facing towards the structure. I.e. I could not enable flying if I was walking sideways or backwards at the slab/water interface.

Expectation:
To just simply walk through it with no problems.

What happened:
Once I touched the water of the structure, I started flying.

Steps:
1. Build the simple structure shown in the attachment.
2. Walk through it.
3. Flying mode will be enabled automatically.