Error on login

Sometimes, when user tries to login, his minecraft client crashes saing Internal Server Error.
This is what console shows:

Code:

2011-01-27 11:46:29 [WARNING] Failed to handle packet: java.lang.NullPointerException
java.lang.NullPointerException
    at net.minecraft.server.ItemStack.d(ItemStack.java:89)
    at net.minecraft.server.ItemStack.f(ItemStack.java:97)
    at net.minecraft.server.InventoryPlayer.a(InventoryPlayer.java:117)
    at net.minecraft.server.EntityItem.b(EntityItem.java:192)
    at net.minecraft.server.EntityPlayer.j(EntityPlayer.java:157)
    at net.minecraft.server.EntityPlayer.o(EntityPlayer.java:149)
    at net.minecraft.server.EntityPlayerMP.o(EntityPlayerMP.java:166)
    at net.minecraft.server.EntityLiving.b_(EntityLiving.java:225)
    at net.minecraft.server.EntityPlayer.b_(EntityPlayer.java:61)
    at net.minecraft.server.EntityPlayerMP.n(EntityPlayerMP.java:114)
    at net.minecraft.server.NetServerHandler.a(NetServerHandler.java:176)
    at net.minecraft.server.Packet10Flying.a(SourceFile:126)
    at net.minecraft.server.NetworkManager.a(SourceFile:232)
    at net.minecraft.server.NetServerHandler.a(NetServerHandler.java:65)
    at net.minecraft.server.NetworkListenThread.a(SourceFile:104)
    at net.minecraft.server.MinecraftServer.h(MinecraftServer.java:261)
    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:197)
    at net.minecraft.server.ThreadServerApplication.run(SourceFile:512)