Steps (using the multiplayer server.jar):
1. Make sure you are in Creative mode.
2. Build a Nether portal in world spawn loaded chunks.
3. Enter the Nether Portal.
4. Don't break an Obsidian block (of the portal frame).

Issue:
Because in creative you initially have no portal cool down and can go directly into the Nether portal and teleport, there is an issue I encountered. I assume the issue is because I have a 0 portal cool down (as it is supposed to be in Creative mode) and you end up going back and forth between dimensions with no escape. The only ways out are if you can break part of the nether portal frame before you teleport again, get /kill'ed or /tp'ed.

I was not able to recreate using "Open to LAN" on single player or on single player mode (not open to LAN), or localhost servers. My server is hosted through a company and I am connecting using the DNS URL.

Causes a "Player moved too quickly" error in console.

Notes: This server is vanilla, and it is my current world for my soon to be public server. The Nether Portal is in world spawn loaded chunks. This issue only happened when the Nether was generated prior to entering. The first time I generated it, the issue didn't happen until I went from the Nether into the Overworld

The attached crash report was obtained while being "stuck" teleporting.