The bug is that it is possible to start flying in the creative game mode without having to press the jump key twice. Just press the inventory key in correct sequence.

How to start flying by using inventory key:

• hold down jump button and when in air, press inventory key
• wait until player is on ground (still holding jump)
• close the inventory and when the player is in air again, quickly press the inventory key twice (still holding jump)

If done correctly, You should be now flying without need to press the jump button twice.