Assign errors happen when you attempt to assign a new value to a built-in kdb function.

Notice I cannot assign a new value to asc as it's a reserved word. I can however reassign the value of functions in the .Q namespace that kdb provides. On the rare occasion kx add a new function, e.g. they added ema (exponential moving average) as a built-in, this can break existing code if anyone had added their own ema function.