So I am developping a plugin using Spigot's api obviously and while I was trying to remove and Item after it being used i run over this exception
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[Server thread/FATAL]: Error executing task
java.util.concurrent.ExecutionException: java.lang.AssertionError: TRAP
at java.util.concurrent.FutureTask.report(Unknown Source) ~[?:1.8.0_111]
at java.util.concurrent.FutureTask.get(Unknown Source) ~[?:1.8.0_111]
at net.minecraft.server.v1_11_R1.SystemUtils.a(SourceFile:47) [spigot.jar:git-Spigot-7d78b81-e2a288c]
at net.minecraft.server.v1_11_R1.MinecraftServer.D(MinecraftServer.java:739) [spigot.jar:git-Spigot-7d78b81-e2a288c]
at net.minecraft.server.v1_11_R1.DedicatedServer.D(DedicatedServer.java:399) [spigot.jar:git-Spigot-7d78b81-e2a288c]
at net.minecraft.server.v1_11_R1.MinecraftServer.C(MinecraftServer.java:675) [spigot.jar:git-Spigot-7d78b81-e2a288c]
at net.minecraft.server.v1_11_R1.MinecraftServer.run(MinecraftServer.java:574) [spigot.jar:git-Spigot-7d78b81-e2a288c]
at java.lang.Thread.run(Unknown Source) [?:1.8.0_111]
Caused by: java.lang.AssertionError: TRAP
at net.minecraft.server.v1_11_R1.ItemStack.F(ItemStack.java:86) ~[spigot.jar:git-Spigot-7d78b81-e2a288c]
at net.minecraft.server.v1_11_R1.ItemStack.setCount(ItemStack.java:814) ~[spigot.jar:git-Spigot-7d78b81-e2a288c]
at net.minecraft.server.v1_11_R1.PlayerInteractManager.a(PlayerInteractManager.java:403) ~[spigot.jar:git-Spigot-7d78b81-e2a288c]
at net.minecraft.server.v1_11_R1.PlayerConnection.a(PlayerConnection.java:957) ~[spigot.jar:git-Spigot-7d78b81-e2a288c]
at net.minecraft.server.v1_11_R1.PacketPlayInBlockPlace.a(PacketPlayInBlockPlace.java:26) ~[spigot.jar:git-Spigot-7d78b81-e2a288c]
at net.minecraft.server.v1_11_R1.PacketPlayInBlockPlace.a(PacketPlayInBlockPlace.java:1) ~[spigot.jar:git-Spigot-7d78b81-e2a288c]
at net.minecraft.server.v1_11_R1.PlayerConnectionUtils$1.run(SourceFile:13) ~[spigot.jar:git-Spigot-7d78b81-e2a288c]
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) ~[?:1.8.0_111]
at java.util.concurrent.FutureTask.run(Unknown Source) ~[?:1.8.0_111]
at net.minecraft.server.v1_11_R1.SystemUtils.a(SourceFile:46) ~[spigot.jar:git-Spigot-7d78b81-e2a288c]
... 5 more
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It happens after all the things are done, and everything in my code executes sucessfully, so I am guessing it is coming from the API. I am currently running the latest build offered by the BuildTools. Hope it can helps you if it really coming from Spigot, keep up the good work! If you want more details or have any questions feel free to ask!