One that always irritates me -
Myth: The Mayans were one big happy empire, just like the Romans or Persians or Chinese or something; i.e., there was a "Mayan Empire".
I've even heard professional historians and history professors call the Mayans an "Empire" (although usually such academics specialize in non-MesoAmerican history...), although to my understanding the Mayans were just a bunch of city-states and although some of these city-states exerted considerable influence and domination at times, they never were able to actually unite all the Mayans under one "Empire".
I acutally see Greece (Pre-Macedonian) referred to the same way pretty often. I think a lot of people assume if an older civilization was "great" it must have been an empire.