This is a weird bug that has to do with block placement. Step 1: Be in Creative Mode, and fly onto a platform 1 block thick that is at least 3 blocks in the air.

Step 2: Fly down and land on the platform. DO NOT stop flying. DO NOT let go of SHIFT. 

Step 3: Hold down Right Click for a few seconds, aiming at the side of the platform. You will place blocks underneath the platform.

There is a video showing the glitch in more detail.