This has happened on every snapshot since the beginning few. We thought it was fixed in version 15w50a, but it wasn't.

Basically, on our 1.9 snapshot server running version 16w03a (but this happens on all versions) if a player goes through the portal, they have a chance of getting stuck, where you get stuck in downloading terrain and you get sent through the worlds. As soon as you load the terrain for the over world, you go back to the nether and visa versa.

The only way to escape the trap, is by having another player tp you, or disconnecting. But when you disconnect and relog, you spawn in a random location (this was a bug that was supposedly fixed)

This can be replicated by multiple users from around the globe. The server is hosted on 10GB of ram and doesn't lag a bit, this is a bug.