This is printed to console:

[19:15:25] [Server thread/INFO]: Attempted to place a tile entity (net.minecraft.server.v1_9_R1.TileEntitySkull@79da0d0f) at 0,0,0 (BEDROCK) where there was no entity tile!
[19:15:25] [Server thread/INFO]: Chunk coordinates: -288,-896
[19:15:25] [Server thread/WARN]: java.lang.Exception
[19:15:25] [Server thread/WARN]: at net.minecraft.server.v1_9_R1.Chunk.a(Chunk.java:724)
[19:15:25] [Server thread/WARN]: at net.minecraft.server.v1_9_R1.Chunk.a(Chunk.java:702)
[19:15:25] [Server thread/WARN]: at net.minecraft.server.v1_9_R1.ChunkRegionLoader.loadEntities(ChunkRegionLoader.java:373)
[19:15:25] [Server thread/WARN]: at org.bukkit.craftbukkit.v1_9_R1.chunkio.ChunkIOProvider.callStage2(ChunkIOProvider.java:42)
[19:15:25] [Server thread/WARN]: at org.bukkit.craftbukkit.v1_9_R1.chunkio.ChunkIOProvider.callStage2(ChunkIOProvider.java:1)
[19:15:25] [Server thread/WARN]: at org.bukkit.craftbukkit.v1_9_R1.util.AsynchronousExecutor.skipQueue(AsynchronousExecutor.java:337)
[19:15:25] [Server thread/WARN]: at org.bukkit.craftbukkit.v1_9_R1.util.AsynchronousExecutor.getSkipQueue(AsynchronousExecutor.java:295)
[19:15:25] [Server thread/WARN]: at org.bukkit.craftbukkit.v1_9_R1.chunkio.ChunkIOExecutor.syncChunkLoad(ChunkIOExecutor.java:16)
[19:15:25] [Server thread/WARN]: at net.minecraft.server.v1_9_R1.ChunkProviderServer.getChunkAt(ChunkProviderServer.java:131)
[19:15:25] [Server thread/WARN]: at net.minecraft.server.v1_9_R1.ChunkProviderServer.getChunkAt(ChunkProviderServer.java:113)
[19:15:25] [Server thread/WARN]: at net.minecraft.server.v1_9_R1.World.getChunkAt(World.java:298)
[19:15:25] [Server thread/WARN]: at net.minecraft.server.v1_9_R1.ChunkCache.<init>(SourceFile:36)
[19:15:25] [Server thread/WARN]: at net.minecraft.server.v1_9_R1.NavigationAbstract.a(SourceFile:110)
[19:15:25] [Server thread/WARN]: at net.minecraft.server.v1_9_R1.Navigation.a(SourceFile:52)
[19:15:25] [Server thread/WARN]: at net.minecraft.server.v1_9_R1.Navigation.a(SourceFile:75)
[19:15:25] [Server thread/WARN]: at net.minecraft.server.v1_9_R1.PathfinderGoalMeleeAttack.a(SourceFile:42)
[19:15:25] [Server thread/WARN]: at net.minecraft.server.v1_9_R1.PathfinderGoalSelector.a(SourceFile:94)
[19:15:25] [Server thread/WARN]: at net.minecraft.server.v1_9_R1.EntityInsentient.doTick(EntityInsentient.java:642)
[19:15:25] [Server thread/WARN]: at net.minecraft.server.v1_9_R1.EntityLiving.n(EntityLiving.java:1903)
[19:15:25] [Server thread/WARN]: at net.minecraft.server.v1_9_R1.EntityInsentient.n(EntityInsentient.java:487)
[19:15:25] [Server thread/WARN]: at net.minecraft.server.v1_9_R1.EntityMonster.n(EntityMonster.java:24)
[19:15:25] [Server thread/WARN]: at net.minecraft.server.v1_9_R1.EntityZombie.n(EntityZombie.java:175)
[19:15:25] [Server thread/WARN]: at net.minecraft.server.v1_9_R1.EntityLiving.m(EntityLiving.java:1759)
[19:15:25] [Server thread/WARN]: at net.minecraft.server.v1_9_R1.EntityInsentient.m(EntityInsentient.java:243)
[19:15:25] [Server thread/WARN]: at net.minecraft.server.v1_9_R1.EntityMonster.m(EntityMonster.java:28)
[19:15:25] [Server thread/WARN]: at net.minecraft.server.v1_9_R1.EntityZombie.m(EntityZombie.java:232)
[19:15:25] [Server thread/WARN]: at net.minecraft.server.v1_9_R1.World.entityJoinedWorld(World.java:1579)
[19:15:25] [Server thread/WARN]: at net.minecraft.server.v1_9_R1.World.g(World.java:1554)
[19:15:25] [Server thread/WARN]: at net.minecraft.server.v1_9_R1.World.tickEntities(World.java:1394)
[19:15:25] [Server thread/WARN]: at net.minecraft.server.v1_9_R1.WorldServer.tickEntities(WorldServer.java:627)
[19:15:25] [Server thread/WARN]: at net.minecraft.server.v1_9_R1.MinecraftServer.D(MinecraftServer.java:792)
[19:15:25] [Server thread/WARN]: at net.minecraft.server.v1_9_R1.DedicatedServer.D(DedicatedServer.java:400)
[19:15:25] [Server thread/WARN]: at net.minecraft.server.v1_9_R1.MinecraftServer.C(MinecraftServer.java:660)
[19:15:25] [Server thread/WARN]: at net.minecraft.server.v1_9_R1.MinecraftServer.run(MinecraftServer.java:559)
[19:15:25] [Server thread/WARN]: at java.lang.Thread.run(Thread.java:745)