Video of glitch can be seen at
https://www.youtube.com/watch?v=PLGHra49pCM

What I expected to happen was...:
Nothing special; this just happens when you run a world twice in a row (relogging} within 15w36d.

What actually happened was...:
Wherever you are, the chunks around you start constantly changing at a high speed - they appear to be changing into different biomes with different mobs appearing within them. However, if you check your biome using F3, the biome name is the same even while the rest is going crazy.

Steps to Reproduce:
Create or open a world, quit to title, and then reload world from list of worlds. Only started happening after I started using 15w36d. To see the effect, you can watch the video of it that I posted at https://www.youtube.com/watch?v=PLGHra49pCM .