CRAFTBOOK-2476 - Reported by paulj1x

CraftBook 3.6b3
Craftbukkit 1.5.1-R0.1 (Build #02718)

On Startup, without anybody logging it, see the following errors:
2013-03-27 22:59:54 [WARNING] [CraftBook] Task #2659 for CraftBook v1886-d04eb23 generated an exception
java.lang.NullPointerException
at org.bukkit.craftbukkit.v1_5_R2.block.CraftSign.(CraftSign.java:17)
at org.bukkit.craftbukkit.v1_5_R2.block.CraftBlock.getState(CraftBlock.java:243)
at com.sk89q.craftbook.bukkit.util.BukkitUtil.toChangedSign(BukkitUtil.java:73)
at com.sk89q.craftbook.circuits.ic.ICMechanic$1.run(ICMechanic.java:78)
at org.bukkit.craftbukkit.v1_5_R2.scheduler.CraftTask.run(CraftTask.java:53)
at org.bukkit.craftbukkit.v1_5_R2.scheduler.CraftScheduler.mainThreadHeartbeat(CraftScheduler.java:344)
at net.minecraft.server.v1_5_R2.MinecraftServer.r(MinecraftServer.java:510)
at net.minecraft.server.v1_5_R2.DedicatedServer.r(DedicatedServer.java:225)
at net.minecraft.server.v1_5_R2.MinecraftServer.q(MinecraftServer.java:474)
at net.minecraft.server.v1_5_R2.MinecraftServer.run(MinecraftServer.java:407)
at net.minecraft.server.v1_5_R2.ThreadServerApplication.run(SourceFile:573)
2013-03-27 22:59:54 [WARNING] [CraftBook] Task #2658 for CraftBook v1886-d04eb23 generated an exception
java.lang.NullPointerException
at org.bukkit.craftbukkit.v1_5_R2.block.CraftSign.(CraftSign.java:17)
at org.bukkit.craftbukkit.v1_5_R2.block.CraftBlock.getState(CraftBlock.java:243)
at com.sk89q.craftbook.bukkit.util.BukkitUtil.toChangedSign(BukkitUtil.java:73)
at com.sk89q.craftbook.circuits.ic.ICMechanic$1.run(ICMechanic.java:78)
at org.bukkit.craftbukkit.v1_5_R2.scheduler.CraftTask.run(CraftTask.java:53)
at org.bukkit.craftbukkit.v1_5_R2.scheduler.CraftScheduler.mainThreadHeartbeat(CraftScheduler.java:344)
at net.minecraft.server.v1_5_R2.MinecraftServer.r(MinecraftServer.java:510)
at net.minecraft.server.v1_5_R2.DedicatedServer.r(DedicatedServer.java:225)
at net.minecraft.server.v1_5_R2.MinecraftServer.q(MinecraftServer.java:474)
at net.minecraft.server.v1_5_R2.MinecraftServer.run(MinecraftServer.java:407)
at net.minecraft.server.v1_5_R2.ThreadServerApplication.run(SourceFile:573)
2013-03-27 22:59:54 [WARNING] [CraftBook] Task #2655 for CraftBook v1886-d04eb23 generated an exception
java.lang.NullPointerException
at org.bukkit.craftbukkit.v1_5_R2.block.CraftSign.(CraftSign.java:17)
at org.bukkit.craftbukkit.v1_5_R2.block.CraftBlock.getState(CraftBlock.java:243)
at com.sk89q.craftbook.bukkit.util.BukkitUtil.toChangedSign(BukkitUtil.java:73)
at com.sk89q.craftbook.circuits.ic.ICMechanic$1.run(ICMechanic.java:78)
at org.bukkit.craftbukkit.v1_5_R2.scheduler.CraftTask.run(CraftTask.java:53)
at org.bukkit.craftbukkit.v1_5_R2.scheduler.CraftScheduler.mainThreadHeartbeat(CraftScheduler.java:344)
at net.minecraft.server.v1_5_R2.MinecraftServer.r(MinecraftServer.java:510)
at net.minecraft.server.v1_5_R2.DedicatedServer.r(DedicatedServer.java:225)
at net.minecraft.server.v1_5_R2.MinecraftServer.q(MinecraftServer.java:474)
at net.minecraft.server.v1_5_R2.MinecraftServer.run(MinecraftServer.java:407)
at net.minecraft.server.v1_5_R2.ThreadServerApplication.run(SourceFile:573)
2013-03-27 22:59:54 [WARNING] [CraftBook] Task #2656 for CraftBook v1886-d04eb23 generated an exception
java.lang.NullPointerException
at org.bukkit.craftbukkit.v1_5_R2.block.CraftSign.(CraftSign.java:17)
at org.bukkit.craftbukkit.v1_5_R2.block.CraftBlock.getState(CraftBlock.java:243)
at com.sk89q.craftbook.bukkit.util.BukkitUtil.toChangedSign(BukkitUtil.java:73)
at com.sk89q.craftbook.circuits.ic.ICMechanic$1.run(ICMechanic.java:78)
at org.bukkit.craftbukkit.v1_5_R2.scheduler.CraftTask.run(CraftTask.java:53)
at org.bukkit.craftbukkit.v1_5_R2.scheduler.CraftScheduler.mainThreadHeartbeat(CraftScheduler.java:344)
at net.minecraft.server.v1_5_R2.MinecraftServer.r(MinecraftServer.java:510)
at net.minecraft.server.v1_5_R2.DedicatedServer.r(DedicatedServer.java:225)
at net.minecraft.server.v1_5_R2.MinecraftServer.q(MinecraftServer.java:474)
at net.minecraft.server.v1_5_R2.MinecraftServer.run(MinecraftServer.java:407)
at net.minecraft.server.v1_5_R2.ThreadServerApplication.run(SourceFile:573)
2013-03-27 22:59:54 [WARNING] [CraftBook] Task #2654 for CraftBook v1886-d04eb23 generated an exception
java.lang.NullPointerException
at org.bukkit.craftbukkit.v1_5_R2.block.CraftSign.(CraftSign.java:17)
at org.bukkit.craftbukkit.v1_5_R2.block.CraftBlock.getState(CraftBlock.java:243)
at com.sk89q.craftbook.bukkit.util.BukkitUtil.toChangedSign(BukkitUtil.java:73)
at com.sk89q.craftbook.circuits.ic.ICMechanic$1.run(ICMechanic.java:78)
at org.bukkit.craftbukkit.v1_5_R2.scheduler.CraftTask.run(CraftTask.java:53)
at org.bukkit.craftbukkit.v1_5_R2.scheduler.CraftScheduler.mainThreadHeartbeat(CraftScheduler.java:344)
at net.minecraft.server.v1_5_R2.MinecraftServer.r(MinecraftServer.java:510)
at net.minecraft.server.v1_5_R2.DedicatedServer.r(DedicatedServer.java:225)
at net.minecraft.server.v1_5_R2.MinecraftServer.q(MinecraftServer.java:474)
at net.minecraft.server.v1_5_R2.MinecraftServer.run(MinecraftServer.java:407)
at net.minecraft.server.v1_5_R2.ThreadServerApplication.run(SourceFile:573)
2013-03-27 22:59:54 [WARNING] [CraftBook] Task #2657 for CraftBook v1886-d04eb23 generated an exception
java.lang.NullPointerException
at org.bukkit.craftbukkit.v1_5_R2.block.CraftSign.(CraftSign.java:17)
at org.bukkit.craftbukkit.v1_5_R2.block.CraftBlock.getState(CraftBlock.java:243)
at com.sk89q.craftbook.bukkit.util.BukkitUtil.toChangedSign(BukkitUtil.java:73)
at com.sk89q.craftbook.circuits.ic.ICMechanic$1.run(ICMechanic.java:78)
at org.bukkit.craftbukkit.v1_5_R2.scheduler.CraftTask.run(CraftTask.java:53)
at org.bukkit.craftbukkit.v1_5_R2.scheduler.CraftScheduler.mainThreadHeartbeat(CraftScheduler.java:344)
at net.minecraft.server.v1_5_R2.MinecraftServer.r(MinecraftServer.java:510)
at net.minecraft.server.v1_5_R2.DedicatedServer.r(DedicatedServer.java:225)
at net.minecraft.server.v1_5_R2.MinecraftServer.q(MinecraftServer.java:474)
at net.minecraft.server.v1_5_R2.MinecraftServer.run(MinecraftServer.java:407)
at net.minecraft.server.v1_5_R2.ThreadServerApplication.run(SourceFile:573)

Comment by me4502

That is due to a bug in bukkit. If you use Spigot, the latest builds of it fix that.