@fuj1n T.T I found it duplicate after I uploaded this ticket... WHY AM I SO UNLUCKY?!
This updates the redstone wire itself again, again and again. Ahh, this is annoying.
@Mustek Ok, thank you.
Summary : This happens when you use Bukkit map to Vanilla SMP server, and this is because of incompatibility between unmodded 12w02b and bukkit. The bukkit sets "CanPickUpLoot" NBT Tag's value to 1 (not 0 the default), and 12w02b doesn't support that, so this occurs. (sorry for my bad english if i used it)
Congratulations, you are the 101th person who can't search and see that this has already been resolved for the next snapshot. You win absolutely nothing!
Quoted [Mod] Mustek 's reply.
And that's why you have to ask Mod developer first.
It's what I already said lol.
Don't matter.
When will the item texture of the hopper be added?
To fix this bug, Minecraft must check whether the slot selected in MouseDown and MouseUp are the same, or a bug like this happens.
Right-click that comparator and that bug would be enabled.
Sometimes it happens, as well as other versions do.
WIP = Work In Progress
Started from 1.3.1, The minecraft always open a server even if singleplayer, and servers don't support that kinds of particle. So it's NOT a bug.
This is a duplication of https://mojang.atlassian.net/browse/MC-5998
The spawner cart is not available without using 3rd party software or mods, so it might be invalid ticket.