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.