Nether Portals briefly teleport the player to their starting location, but in the other dimension. If there is fire or lava at that location then the player takes fire damage after portal invulnerability has ended. This is accompanied by massive server lag / desync in MP and massive frame lag in SP. This is most likely caused by the anti-cheat (speed hacking) engine teleporting the player back to the original location.
e.g.
Overworld portal at (100,64,100)
Lava at (100, 64, 100) in nether
Overworld ==> Nether:
Fire damage (after portal invulnerability has ended)
Note:
In 1.9 pre-4 this issue was partly fixed by the adding of portal invulnerability. However this workaround is temporary for the 1.9 releaseat best as you still load the far-away chunks, catch fire and so on. The real underlying issue is the anti-speed hacking engine or whatever it is that produces those <PLAYER> moved too quickly messages being too trigger-happy.
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As an unrelated note, I'm not sure this has anything to do with the position bug MC-89928, as portals with lava lakes at the corresponding coordinates no longer set players on fire in 1.9-pre2.
@Fenhl this may be due to some type of fire resistance or invulnerability. It makes no sense that you take a tick of suffocation but not lava damage unless this was the case...
I've noticed that my admin noted that most of the "moving too fast" errors had disappeared from the console in the most recent Pre-Release versions, EXCEPT for when people were traveling through the portals... My question: is this, like suggested on other bugs close to this (MC-89928, MC-90605 and maybe others), could this be an issue with the server saying "you moved too fast from coordinate "x" to "x/8" (or conversely from x to x*8) and temporarily teleporting you back? I'm half wondering if half of these bugs that have been 'patched and closed' are actually useless code that have acted as bandaids for an over-active anti-cheat measure? Sorry if the logic in my mind is not making sense, but if all of these fixes since 15w42a have been fixing the end result of the bugs instead of what's causing them, won't the remaining cause still be the same and cause overall instability in the system, let alone server resources (loading unneeded chunks, network traffic, etc)?
I'll give a reason my mind says MC-90605 was happening that relates to this bug: You teleport to the dimension, anticheat tries to put you back, something added in 15w42+ puts you back in the portal, next thing you know you're being transferred back to the previous dimension because you entered and then left the portal then came back (all by the servers doing), and you get looped back because the cooldown on the nether portal looping is now broken by you entering and leaving then reentering (read above, but the servers doing). Now it was fixed in 16w06 by adding a cooldown. — AGAIN, this is fixing the result, not what caused the result.
My message to Mobius... look into, at least for a second, the root of the issues. Which I believe at this point the anti-cheat measure saying you moved too fast. Then you could probably remove all of the added bloat of correcting a bug that has cascaded into everything from changing dimensions to portal looping by breaking preexisting mechanics that were designed to prevent this in previous versions of the game.
https://youtu.be/oRu6rf4xNt8
More proof the player is actually there, server side. I think this one is quite useful as it shows that the player actually dies at that location.
I have watched James Koon's videos and did some of my own testing, and did some reading over the MCP 9.31 code. Since entire teleportation and changing dimension code is only executed server side, and client is not informed about changing of the coordinates, it might be the case, that the player enters the other dimension at a correct location (A), then client updates player position to (8*A or A/8), clients last known location (for one tick), then server decides to move the player back ('Player moved wrongly' message). It might be just client server dissynchronization issue. It is visible on James Koon's videos, that from the other account perspective (thus not linked with the client of the first account) other account lands on the portal, then moves via clients request to the old coords and then is moved back by the server the third time.
Glad someone actually looked into this... hopefully someone at Mojang can fix this, I'm tired of policing nether portals on my server. 😃
Yep - happend to Xisuma on his stream. crashed the game while going thrgouh the portal and the server were not able to correct his position ('moved wrongly' massage) and end up in the exact coords of the overworld, but in the nether.
Erik Broes
Taking more than one tick's worth of damage is also possible in multiplayer, please update the description accordingly.