Could not pass event NPCRightClickEvent to Citizens v2.0.17-SNAPSHOT (build 1318)
org.bukkit.event.EventException
at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:310) ~[minecraft_server.jar:git-Spigot-90839e6-8e5eab2]
at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62) ~[minecraft_server.jar:git-Spigot-90839e6-8e5eab2]
at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:502) [minecraft_server.jar:git-Spigot-90839e6-8e5eab2]
at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:487) [minecraft_server.jar:git-Spigot-90839e6-8e5eab2]
at net.citizensnpcs.EventListen.onPlayerInteractEntity(EventListen.java:386) [citizens-2.0.17-SNAPSHOT.jar:?]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_77]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_77]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_77]
at java.lang.reflect.Method.invoke(Method.java:498) ~[?:1.8.0_77]
at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:306) [minecraft_server.jar:git-Spigot-90839e6-8e5eab2]
at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62) [minecraft_server.jar:git-Spigot-90839e6-8e5eab2]
at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:502) [minecraft_server.jar:git-Spigot-90839e6-8e5eab2]
at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:487) [minecraft_server.jar:git-Spigot-90839e6-8e5eab2]
at net.minecraft.server.v1_9_R1.PlayerConnection.a(PlayerConnection.java:1532) [minecraft_server.jar:git-Spigot-90839e6-8e5eab2]
at net.minecraft.server.v1_9_R1.PacketPlayInUseEntity.a(SourceFile:68) [minecraft_server.jar:git-Spigot-90839e6-8e5eab2]
at net.minecraft.server.v1_9_R1.PacketPlayInUseEntity.a(SourceFile:12) [minecraft_server.jar:git-Spigot-90839e6-8e5eab2]
at net.minecraft.server.v1_9_R1.PlayerConnectionUtils$1.run(SourceFile:13) [minecraft_server.jar:git-Spigot-90839e6-8e5eab2]
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [?:1.8.0_77]
at java.util.concurrent.FutureTask.run(FutureTask.java:266) [?:1.8.0_77]
at net.minecraft.server.v1_9_R1.SystemUtils.a(SourceFile:45) [minecraft_server.jar:git-Spigot-90839e6-8e5eab2]
at net.minecraft.server.v1_9_R1.MinecraftServer.D(MinecraftServer.java:721) [minecraft_server.jar:git-Spigot-90839e6-8e5eab2]
at net.minecraft.server.v1_9_R1.DedicatedServer.D(DedicatedServer.java:400) [minecraft_server.jar:git-Spigot-90839e6-8e5eab2]
at net.minecraft.server.v1_9_R1.MinecraftServer.C(MinecraftServer.java:660) [minecraft_server.jar:git-Spigot-90839e6-8e5eab2]
at net.minecraft.server.v1_9_R1.MinecraftServer.run(MinecraftServer.java:559) [minecraft_server.jar:git-Spigot-90839e6-8e5eab2]
at java.lang.Thread.run(Thread.java:745) [?:1.8.0_77]

Caused by: java.lang.IllegalArgumentException: Potion Type is not upgradable
at org.apache.commons.lang.Validate.isTrue(Validate.java:136) ~[minecraft_server.jar:git-Spigot-90839e6-8e5eab2]
at org.bukkit.potion.PotionData.(PotionData.java:23) ~[minecraft_server.jar:git-Spigot-90839e6-8e5eab2]
at org.bukkit.craftbukkit.v1_9_R1.potion.CraftPotionUtil.toBukkit(CraftPotionUtil.java:85) ~[minecraft_server.jar:git-Spigot-90839e6-8e5eab2]
at org.bukkit.craftbukkit.v1_9_R1.inventory.CraftMetaPotion.getBasePotionData(CraftMetaPotion.java:159) ~[minecraft_server.jar:git-Spigot-90839e6-8e5eab2]
at regalowl.hyperconomy.bukkit.BukkitCommon.getSerializableItemStack(BukkitCommon.java:441) ~[?:?]
at regalowl.hyperconomy.bukkit.BukkitCommon.getInventory(BukkitCommon.java:265) ~[?:?]
at regalowl.hyperconomy.bukkit.BukkitConnector.getInventory(BukkitConnector.java:358) ~[?:?]
at regalowl.hyperconomy.account.HyperPlayer.getInventory(HyperPlayer.java:274) ~[?:?]
at regalowl.hyperconomy.tradeobject.ComponentTradeItem.getSellPrice(ComponentTradeItem.java:44) ~[?:?]
at regalowl.hyperconomy.tradeobject.BasicTradeObject.getSellPriceWithTax(BasicTradeObject.java:598) ~[?:?]
at grokswell.hypermerchant.ShopMenu.loadPage(ShopMenu.java:200) ~[?:?]
at grokswell.hypermerchant.ShopMenu.(ShopMenu.java:116) ~[?:?]
at grokswell.hypermerchant.HyperMerchantTrait.onRightClick(HyperMerchantTrait.java:367) ~[?:?]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_77]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_77]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_77]
at java.lang.reflect.Method.invoke(Method.java:498) ~[?:1.8.0_77]
at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:306) ~[minecraft_server.jar:git-Spigot-90839e6-8e5eab2]
... 24 more

This currently happens with potions of: Invisibility, Fire Resistance, Slowness, Water Breathing, Weakness, Luck, Awkward, Uncraftable, Water Bottle, Mundane and Thick potions.
There may be more but these are the ones I've checked.

Also using hypermerchant-1.6-dev, with citizens-2.0.17-SNAPSHOT as displayed in the stack trace.

This may also just be an outdated Spigot. Try updating to the latest Spigot to see if this is resolved. I reported a very similar issue to the Spigot team which was resolved a day or two ago.

RegalOwl is right. This doesn't seem to be an issue for me with latest spigot.

This looks to me like a HyperMerchant issue, and I will take a closer look this evening

Sorry for the late response, this was indeed an issue with the version of Spigot. This is all fixed in the "at the moment" latest version of Spigot.