Reference -> https://mojang.atlassian.net/browse/MC-15058
With a 1.5.1 world save, the new snapshot will not allow any players into the game, offering a simple "End of stream" when attempting to log in.
However, when using a brand new world save, generated by the snapshot, the end of stream doesn't occur. Therefore I can reason that it must not be something present in anything 1.5.1 or before.
Let's bear in mind that this world save has existed since the early 12w snapshots, so it's nothing with the save itself.
This issue was marked as fixed, but it most certainly is still an issue.