When you hit a mob (like Zombie) with an item which has a negative looting enchantment, it gives an error message:
Error message
[18:11:18] [Server thread/FATAL]: Error executing task java.util.concurrent.ExecutionException: java.lang.IllegalArgumentException: bound must be positive at java.util.concurrent.FutureTask.report(FutureTask.java:122) ~[?:1.8.0_25] at java.util.concurrent.FutureTask.get(FutureTask.java:192) ~[?:1.8.0_25] at g.a(SourceFile:61) [1.8.4.jar:?] at net.minecraft.server.MinecraftServer.B(SourceFile:579) [1.8.4.jar:?] at net.minecraft.server.MinecraftServer.A(SourceFile:535) [1.8.4.jar:?] at bpo.A(SourceFile:152) [1.8.4.jar:?] at net.minecraft.server.MinecraftServer.run(SourceFile:451) [1.8.4.jar:?] at java.lang.Thread.run(Thread.java:745) [?:1.8.0_25] Caused by: java.lang.IllegalArgumentException: bound must be positive at java.util.Random.nextInt(Random.java:388) ~[?:1.8.0_25] at vw.b(SourceFile:175) ~[1.8.4.jar:?] at pr.a(SourceFile:732) ~[1.8.4.jar:?] at we.a(SourceFile:619) ~[1.8.4.jar:?] at pr.a(SourceFile:684) ~[1.8.4.jar:?] at vv.a(SourceFile:53) ~[1.8.4.jar:?] at we.a(SourceFile:207) ~[1.8.4.jar:?] at vw.a(SourceFile:143) ~[1.8.4.jar:?] at wn.f(SourceFile:1045) ~[1.8.4.jar:?] at lf.f(SourceFile:935) ~[1.8.4.jar:?] at lm.a(SourceFile:772) ~[1.8.4.jar:?] at in.a(SourceFile:52) ~[1.8.4.jar:?] at in.a(SourceFile:11) ~[1.8.4.jar:?] at fh$1.run(SourceFile:13) ~[1.8.4.jar:?] at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) ~[?:1.8.0_25] at java.util.concurrent.FutureTask.run(FutureTask.java:266) ~[?:1.8.0_25] at g.a(SourceFile:60) ~[1.8.4.jar:?] ... 5 more
Item command
/give @p stick 1 0 {ench:[{id:16,lvl:32767},{id:21,lvl:32768}]}
Note that looting seems to be the only enchantment which throws an error when it has a negative value