Sorry, I used the google translator.

In 15w35e I have same error.

When I start my client in LAN world and has more than one player, my client has submitted the following error. But does not cause any other problems apparently. Does not disconnect anyone.

Multiplayer using Client:

[19:53:53] [Client thread/FATAL]: Error executing task
java.util.concurrent.ExecutionException: io.netty.util.IllegalReferenceCountException: refCnt: 0
at java.util.concurrent.FutureTask.report(Unknown Source) ~[?:1.8.0_60]
at java.util.concurrent.FutureTask.get(Unknown Source) ~[?:1.8.0_60]
at g.a(SourceFile:46) [15w36c.jar:?]
at azo.as(SourceFile:921) [15w36c.jar:?]
at azo.a(SourceFile:378) [15w36c.jar:?]
at net.minecraft.client.main.Main.main(SourceFile:125) [15w36c.jar:?]
Caused by: io.netty.util.IllegalReferenceCountException: refCnt: 0
at io.netty.buffer.AbstractByteBuf.ensureAccessible(AbstractByteBuf.java:1187) ~[netty-all-4.0.23.Final.jar:4.0.23.Final]
at io.netty.buffer.AbstractByteBuf.checkReadableBytes(AbstractByteBuf.java:1170) ~[netty-all-4.0.23.Final.jar:4.0.23.Final]
at io.netty.buffer.AbstractByteBuf.readByte(AbstractByteBuf.java:570) ~[netty-all-4.0.23.Final.jar:4.0.23.Final]
at em.readByte(SourceFile:663) ~[15w36c.jar:?]
at aql.a(SourceFile:87) ~[15w36c.jar:?]
at aqr.a(SourceFile:981) ~[15w36c.jar:?]
at bhr.a(SourceFile:522) ~[15w36c.jar:?]
at gp.a(SourceFile:68) ~[15w36c.jar:?]
at gp.a(SourceFile:11) ~[15w36c.jar:?]
at fh$1.run(SourceFile:13) ~[15w36c.jar:?]
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) ~[?:1.8.0_60]
at java.util.concurrent.FutureTask.run(Unknown Source) ~[?:1.8.0_60]
at g.a(SourceFile:45) ~[15w36c.jar:?]
... 3 more

Constant loop this message, when you have more than one player present.

When I use the server and has more than one player, the game features random disconnects the clients. And the following message appears in the logs.

Multiplayer using Server:

[16:44:59] [Server thread/INFO]: Done (1,188s)! For help, type "help" or "?"
[16:45:15] User Authenticator #1/INFO: UUID of player X_Pandaa_X is bd114f37-6648-4e7b-bb55-2a42349db45c
[16:45:15] [Server thread/INFO]: X_Pandaa_X[/192.168.25.87:49748] logged in with entity id 210 at (131.54501831916264, 69.0, -12.48687390268749)
[16:45:15] [Server thread/INFO]: X_Pandaa_X joined the game
[16:45:21] User Authenticator #2/INFO: UUID of player martinaovivo is f053ef4f-eb06-4f1c-a64d-ace5ec397f57
[16:45:21] [Server thread/INFO]: martinaovivo[/192.168.25.2:53838] logged in with entity id 315 at (136.99542966206587, 69.0, -15.63518955819433)
[16:45:21] [Server thread/INFO]: martinaovivo joined the game
[16:45:25] Netty Server IO #4/ERROR: io.netty.util.IllegalReferenceCountException: refCnt: 0
[16:45:26] Netty Server IO #4/ERROR: io.netty.util.IllegalReferenceCountException: refCnt: 0
[16:45:26] Netty Server IO #3/ERROR: io.netty.util.IllegalReferenceCountException: refCnt: 0, decrement: 1
[16:45:26] Netty Server IO #4/ERROR: io.netty.util.IllegalReferenceCountException: refCnt: 0, decrement: 1
[16:45:26] Netty Server IO #4/ERROR: java.lang.IndexOutOfBoundsException: readerIndex: 13384 (expected: 0 <= readerIndex <= writerIndex(6692))
[16:45:26] Netty Server IO #4/ERROR: java.lang.IndexOutOfBoundsException: readerIndex: 13384 (expected: 0 <= readerIndex <= writerIndex(6692))
[16:45:26] Netty Server IO #4/ERROR: io.netty.util.IllegalReferenceCountException: refCnt: 0, decrement: 1
[16:45:26] Netty Server IO #4/ERROR: io.netty.util.IllegalReferenceCountException: refCnt: 0
[16:45:26] [Server thread/INFO]: martinaovivo lost connection: TextComponent{text='Disconnected', siblings=[], style=Style{hasParent=false, color=null, bold=null, italic=null, underlined=null, obfuscated=null, clickEvent=null, hoverEvent=null, insertion=null}}
[16:45:26] [Server thread/INFO]: martinaovivo left the game
[16:45:32] User Authenticator #3/INFO: UUID of player martinaovivo is f053ef4f-eb06-4f1c-a64d-ace5ec397f57
[16:45:32] [Server thread/INFO]: martinaovivo[/192.168.25.2:53841] logged in with entity id 522 at (136.99542966206587, 69.0, -15.63518955819433)
[16:45:32] [Server thread/INFO]: martinaovivo joined the game
[16:45:39] Netty Server IO #5/ERROR: io.netty.util.IllegalReferenceCountException: refCnt: 0
[16:45:39] [Server thread/INFO]: martinaovivo lost connection: TextComponent{text='Disconnected', siblings=[], style=Style{hasParent=false, color=null, bold=null, italic=null, underlined=null, obfuscated=null, clickEvent=null, hoverEvent=null, insertion=null}}
[16:45:39] [Server thread/INFO]: martinaovivo left the game
[16:45:43] User Authenticator #4/INFO: UUID of player martinaovivo is f053ef4f-eb06-4f1c-a64d-ace5ec397f57
[16:45:43] [Server thread/INFO]: martinaovivo[/192.168.25.2:53844] logged in with entity id 651 at (135.84540281625428, 69.0, -15.557666446635043)
[16:45:43] [Server thread/INFO]: martinaovivo joined the game
[16:45:52] Netty Server IO #3/ERROR: io.netty.util.IllegalReferenceCountException: refCnt: 0
[16:45:52] [Server thread/INFO]: X_Pandaa_X lost connection: TextComponent{text='Disconnected', siblings=[], style=Style{hasParent=false, color=null, bold=null, italic=null, underlined=null, obfuscated=null, clickEvent=null, hoverEvent=null, insertion=null}}
[16:45:52] [Server thread/INFO]: X_Pandaa_X left the game

My world download:
https://copy.com/YBgKCzf0VmbvrtHS

Thanks guys