Got this today on almost every mob:

[12:02:26] [LayerHopper] YOU HAVE ENCOUNTERED A MOB THAT BREAKS LAYER HOPPER!
[12:02:26] [LayerHopper] MOB NAME: Аукционер Джаксон
[12:02:26] [LayerHopper] MOB GUID: Creature-0-4459-0-687-15659-00000522AF
[12:02:26] [LayerHopper] ZONE: Торговый квартал, Штормград
[12:02:26] [LayerHopper] PLEASE SEND THIS INFORMATION TO KUTANO (OR REPORT ON GITHUB github.com/psynct/LayerHopper)!

My server is Пламегор (EU)

I've mostly gotten this in Booty Bay. Server is Firemaw on EU.
[00:07:13] [LayerHopper] YOU HAVE ENCOUNTERED A MOB THAT BREAKS LAYER HOPPER!
[00:07:13] [LayerHopper] MOB NAME: Kelsey Yance
[00:07:13] [LayerHopper] MOB GUID: Creature-0-4468-0-140-2664-0000052076
[00:07:13] [LayerHopper] ZONE: The Old Port Authority, Stranglethorn Vale
[00:07:13] [LayerHopper] PLEASE SEND THIS INFORMATION TO KUTANO (OR REPORT ON GITHUB github.com/psynct/LayerHopper)!
[00:08:56] [LayerHopper] YOU HAVE ENCOUNTERED A MOB THAT BREAKS LAYER HOPPER!
[00:08:56] [LayerHopper] MOB NAME: Narkk
[00:08:56] [LayerHopper] MOB GUID: Creature-0-4468-0-140-2663-0000052075
[00:08:56] [LayerHopper] ZONE: The Old Port Authority, Stranglethorn Vale
[00:08:56] [LayerHopper] PLEASE SEND THIS INFORMATION TO KUTANO (OR REPORT ON GITHUB github.com/psynct/LayerHopper)!
[00:10:02] [LayerHopper] YOU HAVE ENCOUNTERED A MOB THAT BREAKS LAYER HOPPER!
[00:10:02] [LayerHopper] MOB NAME: First Mate Crazz
[00:10:02] [LayerHopper] MOB GUID: Creature-0-4468-0-140-2490-0000052073
[00:10:02] [LayerHopper] ZONE: Booty Bay, Stranglethorn Vale
[00:10:02] [LayerHopper] PLEASE SEND THIS INFORMATION TO KUTANO (OR REPORT ON GITHUB github.com/psynct/LayerHopper)!

[08:42] [LayerHopper] YOU HAVE ENCOUNTERED A MOB THAT BREAKS LAYER HOPPER!
[08:42] [LayerHopper] MOB NAME: Хозяин таверны Втридорог
[08:42] [LayerHopper] MOB GUID: Creature-0-4459-0-695-6807-00000522B5
[08:42] [LayerHopper] ZONE: Таверна "Старый моряк", Тернистая долина
[08:42] [LayerHopper] PLEASE SEND THIS INFORMATION TO KUTANO (OR REPORT ON GITHUB github.com/psynct/LayerHopper)!
[08:42] [LayerHopper] YOU HAVE ENCOUNTERED A MOB THAT BREAKS LAYER HOPPER!
[08:42] [LayerHopper] MOB NAME: Грингер
[08:42] [LayerHopper] MOB GUID: Creature-0-4459-0-695-2858-00000522B7
[08:42] [LayerHopper] ZONE: Пиратская бухта, Тернистая долина
[08:42] [LayerHopper] PLEASE SEND THIS INFORMATION TO KUTANO (OR REPORT ON GITHUB github.com/psynct/LayerHopper)!
[08:44] [LayerHopper] YOU HAVE ENCOUNTERED A MOB THAT BREAKS LAYER HOPPER!
[08:44] [LayerHopper] MOB NAME: Острозубая бешенка
[08:44] [LayerHopper] MOB GUID: Creature-0-4459-0-695-905-000006CE22
[08:44] [LayerHopper] ZONE: , Тернистая долина
[08:44] [LayerHopper] PLEASE SEND THIS INFORMATION TO KUTANO (OR REPORT ON GITHUB github.com/psynct/LayerHopper)!

Mobname : thulnos
mob guid: creature-0-4468-389-31430-1860-000011A2D7
zone: cleft of shadows (ogrimmar)

I have no idea why these NPCs have IDs outside of the valid ranges. Either there are more than 2 layers on your servers or blizz changed something. I don't really play classic anymore after my guild self destructed so any pull requests if someone can figure out the issue would be appreciated.

Still occurs it seems

[14:38] [LayerHopper] YOU HAVE ENCOUNTERED A MOB THAT BREAKS LAYER HOPPER!
[14:38] [LayerHopper] MOB NAME: Elling Trias
[14:38] [LayerHopper] MOB GUID: Creature-0-4479-0-10-482-00002B578D
[14:38] [LayerHopper] ZONE: Trade District, Stormwind City
[14:38] [LayerHopper] PLEASE SEND THIS INFORMATION TO KUTANO (OR REPORT ON GITHUB github.com/psynct/LayerHopper)!

Seems like it happens if you were in a contested zone or other continent then go to a city afterwards. Not sure.