CraftBukkit Update: This fix (Amaranth's CraftBukkit implementation) has made it into CraftBukkit. So if you're on a Bukkit server with version 1.2.5-R0.1 (Build #02136) or later, you don't need BetterSMP at all - you can still use SMPSlimeFix to further improve slime behaviour though.

This whole issue will most likely be gone by 1.3. Until then, these mods should prove useful to many SMP players.

These mods basically are able to (partially) fix the following SMP-related bugs:

• Entities visually glitching through blocks client-side and bumping you around
  -> Fixed by BetterSMP-Server or BetterSMP-Client
  
• Entites permanently glitching through blocks server-side (seems to occur mostly when reloading chunks)
  -> Fixed by BetterSMP-Server
  
• Slimes are often assigned wrong bounding boxes
  -> Fixed by BetterSMP-Client or SMPSlimeFix
  

Recommended Setups:

• Client: SMPSlimeFix, Server: BetterSMP
  -or-
  
• Client: BetterSMP (does not fix permanent server-side glitches though)
  

Using BetterSMP both on client and server *might* cause issues.

Client Mod: BetterSMP_Client
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-> Put class file into minecraft.jar and delete the META-INF folder

What it does: Corrects the mob positions received from the server to (mostly) prevent mobs from glitching through blocks. Also corrects the bounding box of slimes.

This should fix:

• Mobs glitching out of cells, traps etc. client-side (though it still happens sometimes, depending on the exact setup; full block spaces seem to work better than using fences, slabs, panes etc.). They may still be very glitchy directly after logging in, or when they spawn. This should resolve itself over time
  
• Slime movement in SMP by some extent
  
• maybe some other stuff (though I can't deny there might be new issues too )
  

Server: BetterSMP_Server & BetterSMP_Bukkit
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-> Bukkit: Put jar file into plugins folder
-> Vanilla Server: Put class file into minecraft_server.jar

Bukkit Note: The Vanilla mod had to be rewritten to fit into the Bukkit Plugin system and the plugin ended up being quite hacky. So it may behave slightly differentlly (for better or worse) than the vanilla server mod. Should work fine with all 1.2.x releases.

What it does: Corrects mob positions sent to clients to (mostly) prevent mobs from glitching through blocks. Also prevents them from permanently glitching through blocks on the server side.

This should fix:

• Mobs glitching out of cells, traps etc. client-side (though it still happens sometimes, depending on the exact setup; full block spaces should work better than using fences, slabs, panes etc.). They may still be very glitchy directly after logging in, or when they spawn. This should resolve itself over time
  
• Mobs permanently escaping cells / fenced areas
  
• Minecarts permanently falling through blocks
  
• maybe some other stuff (though I can't deny there might be new issues too )
  

Client Mod: SMPSlimeFix
------------------------------------
-> Put class file into minecraft.jar and delete the META-INF folder

What it does: Corrects the bounding box of slimes spawned in multiplayer.

This should fix:

• Slime movement in SMP by some extent
  

Note: SMPSlimeFix is integrated into BetterSMP-Client. So you should only use SMPSlimeFix if your server is already running BetterSMP-Server.