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
Wear your elytra and look directly upwards
Jump and activate your elytra. You'll see yourself glitch through the floor before you get kicked
Linked issues
Attachments
Comments 3

No, this bug was introduced in the snapshots.
Fixing MC-122354 will make this impossible to experience anyway...
Does this also affect 1.12.2?