Minecraft 14w26b, Minecraft 14w26c, Minecraft 14w27a, Minecraft 14w27b
Server: CentOS 6.5, 32GB RAM,
Java version: 1.7.0_10
Client: Windows 7, 8GB RAM
As of 14w26a/b, you can no longer add fake players to scoreboard teams.
Steps to reproduce:
1. /scoreboard teams add <name>
2. /scoreboard teams join <name> FakePlayer
It will error with 'The entity UUID provided is in an invalid format'.
Just to clarify, you can still add fake players to objectives and modify their score, just like normal. You can also add custom-named entities to teams, though their custom name does not show up if you set the display to 'sidebar'. It reverts to their UUID.