See MC-122354 before you read.

The bug

When you stand still on a block, look directly up and activate your elytra, not only does the internal server kicks you out from your single player world and opens the servers menu, you temporarily glitch through the floor before the kick. When you reload, you spawn right where you were before.
Further testing proves that you don't have to set up a platform and perform the glitch. This can be abused as an x-ray glitch.

How to reproduce

1. Wear your elytra and look directly upwards
2. Jump and activate your elytra. You'll see yourself glitch through the floor before you get kicked