I builded a lift and when I tried to activate the 4th floor nothing happened. I knew why, because I builded some dirt blocks from the 3th floor up to the 4th floor. They were blocking the area for the 3x3 glass blocks. The Server gave me the error list below. When I removed the dirt blocks, the sign worked. I run a MCPC+ Server (1.6.2-R0.2-forge836-B56)

01:24:48 [SEVERE] Could not pass event PlayerInteractEvent to Lift v40
01:24:48 org.bukkit.event.EventException
01:24:48 at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:363)
01:24:48 at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62)
01:24:48 at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:479)
01:24:48 at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:464)
01:24:48 at org.bukkit.craftbukkit.v1_6_R2.event.CraftEventFactory.callPlayerInteractEvent(CraftEventFactory.java:190)
01:24:48 at net.minecraft.item.ItemInWorldManager.func_73078_a(ItemInWorldManager.java:547)
01:24:48 at net.minecraft.network.NetServerHandler.func_72472_a(NetServerHandler.java:881)
01:24:48 at net.minecraft.network.packet.Packet15Place.func_73279_a(Packet15Place.java:79)
01:24:48 at net.minecraft.network.TcpConnection.func_74428_b(TcpConnection.java:464)
01:24:48 at net.minecraft.network.NetServerHandler.func_72570_d(NetServerHandler.java:230)
01:24:48 at net.minecraft.network.NetworkListenThread.func_71747_b(NetworkListenThread.java:54)
01:24:48 at net.minecraft.server.dedicated.DedicatedServerListenThread.func_71747_b(DedicatedServerListenThread.java:34)
01:24:48 at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:872)
01:24:48 at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:318)
01:24:48 at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:741)
01:24:48 at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:625)
01:24:48 at net.minecraft.server.ThreadMinecraftServer.run(ThreadMinecraftServer.java:16)
01:24:48 Caused by: java.lang.NullPointerException
01:24:48 at net.croxis.plugins.lift.BukkitLiftPlayerListener.onPlayerInteract(BukkitLiftPlayerListener.java:80)
01:24:48 at sun.reflect.GeneratedMethodAccessor50.invoke(Unknown Source)
01:24:48 at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
01:24:48 at java.lang.reflect.Method.invoke(Unknown Source)
01:24:48 at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:361)
01:24:48 ... 16 more
01:24:49 [SEVERE] Could not pass event PlayerInteractEvent to Lift v40
01:24:49 org.bukkit.event.EventException
01:24:49 at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:363)
01:24:49 at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62)
01:24:49 at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:479)
01:24:49 at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:464)
01:24:49 at org.bukkit.craftbukkit.v1_6_R2.event.CraftEventFactory.callPlayerInteractEvent(CraftEventFactory.java:190)
01:24:49 at net.minecraft.item.ItemInWorldManager.func_73074_a(ItemInWorldManager.java:167)
01:24:49 at net.minecraft.network.NetServerHandler.func_72510_a(NetServerHandler.java:759)
01:24:49 at net.minecraft.network.packet.Packet14BlockDig.func_73279_a(Packet14BlockDig.java:67)
01:24:49 at net.minecraft.network.TcpConnection.func_74428_b(TcpConnection.java:464)
01:24:50 at net.minecraft.network.NetServerHandler.func_72570_d(NetServerHandler.java:230)
01:24:50 at net.minecraft.network.NetworkListenThread.func_71747_b(NetworkListenThread.java:54)
01:24:50 at net.minecraft.server.dedicated.DedicatedServerListenThread.func_71747_b(DedicatedServerListenThread.java:34)
01:24:50 at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:872)
01:24:50 at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:318)
01:24:50 at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:741)
01:24:50 at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:625)
01:24:50 at net.minecraft.server.ThreadMinecraftServer.run(ThreadMinecraftServer.java:16)
01:24:50 Caused by: java.lang.NullPointerException
01:24:50 at net.croxis.plugins.lift.BukkitLiftPlayerListener.onPlayerInteract(BukkitLiftPlayerListener.java:80)
01:24:50 at sun.reflect.GeneratedMethodAccessor50.invoke(Unknown Source)
01:24:50 at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
01:24:50 at java.lang.reflect.Method.invoke(Unknown Source)
01:24:50 at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:361)
01:24:50 ... 16 more

I found the fix, commited for version 42.