EDIT
When you set the world border to 10 or something...
then going to 30000000 100 30000000
then gamemode 3 you can easily fly into the boundary and give an error
Illegal position, then dumping you to the multiplayer screen
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Can confirm. World barrier doesn't prevent going off the world limits in gamemode 1 either, kicks you off the game and dumps you on the server list.

Since the farlands were removed many versions ago there is no real reason anymore to go so far away from spawn. As soon as a player manages to get there without using teleports or other cheat, we can reopen this ticket.
@searge
Challenge accepted! can i use the top of the nether?
lets get these speed potions ready!
If you go out of the worldborder, the game wil say "Disconnected, Illegal position.
@Butt Man
It would still take you ~135 hours of running with permanent speed II (which would take ~2000 speed potions). So good luck trying that in survival ;P

Confirmed for
1.8.5 However I believe that this is a valid bug because there are these bugs with it:
"[Server thread/WARN]: handleDisconnection() called twice" can happen
Player position is corrupted after that, player can't enter world anymore (might be problematic mainly on servers), I would expect Minecraft to overwrite these invalid coordinates with the spawnpoint coordinates
The "Illegal position" text is not translated and leads to the server list
It can happen that the Client freezes instead of showing the error even thought the launcher says that the player left the world