it does something that warcraft doesn't like for example, if you make a building and do something with it, warcraft may simply decide that it doesn't like said building at ALL and will close down every time you click/build it the same might happen with units too but i have only heard of/seen it happen with buildings
it's like in starcraft when u disable the doodad properties for a terran missile turret sc doesn't like what the sprite ends up as, u look at it POOF it kills ur starcraft.
note: if you're building those buildings from a building (like having a castle build a tower instead of having a peasant do the job) warcraft may not like the pathing and close down.
Who built the unit? It has to do something with it. AND the pathing and the ground texture. Plus (maybe) there is no Build/Birth animation of the unit u r trying to make. That's all I think.