When you use /op (name) on a 15w33a server, the client throws an exception: (Using Emmia as my username.)
Internal Exception: io.netty.handler.codec.DecoderException: com.google.gson.JsonParseException: Don't know how to turn "Emmia" into a Component

To replicate:
1. Host a 15w33a server
2. Log onto the server using your minecraft account
3. /op yourself using the server console
You will be force-disconnected from the server with the above error code.

Related to this, when I was looking at my server list, I got a bunch of error messages in my log with a similar exception.