Activating some redstone components such at pressure plates causes the following error to appear in the logs:

[13:05:05] [Server thread/WARN]: Tried to assign a mutable BlockPos to tick data
...
java.lang.Error: class cj
at ll.a(SourceFile:479) [minecraft_server.jar:?]
at ll.a(SourceFile:472) [minecraft_server.jar:?]
at ajk.f(SourceFile:117) [minecraft_server.jar:?]
at ajk.a(SourceFile:64) [minecraft_server.jar:?]
at rb.T(SourceFile:778) [minecraft_server.jar:?]
at rb.d(SourceFile:721) [minecraft_server.jar:?]
at xj.a(SourceFile:526) [minecraft_server.jar:?]
at xj.m(SourceFile:329) [minecraft_server.jar:?]
at agq.a(SourceFile:1436) [minecraft_server.jar:?]
at ll.a(SourceFile:661) [minecraft_server.jar:?]
at agq.g(SourceFile:1414) [minecraft_server.jar:?]
at agq.k(SourceFile:1305) [minecraft_server.jar:?]
at ll.k(SourceFile:540) [minecraft_server.jar:?]
at net.minecraft.server.MinecraftServer.D(SourceFile:625) [minecraft_server.jar:?]
at kv.D(SourceFile:314) [minecraft_server.jar:?]
at net.minecraft.server.MinecraftServer.C(SourceFile:553) [minecraft_server.jar:?]
at net.minecraft.server.MinecraftServer.run(SourceFile:457) [minecraft_server.jar:?]
at java.lang.Thread.run(Unknown Source) [?:1.7.0_05]

The warning does not appear to affect gameplay.

Tested an iron door activated by a wooden button and wooden pressure plate. The button did not cause the warning to be thrown, but the pressure plate did. The activation of redstone, redstone torches, and repeaters does not appear to throw the warning.

Activating a tripwire causes the warning.

There seem to be numerous causes, primarily around entities. The log was flooded with this message near a generic mob XP farm with an item elevator.