Netica converts equations to an internal form for fast evaluation. Sometimes that means extra functions are invoked that weren't in the original equation; these functions always start with an underscore.
You may receive an error message that indicates there was a problem evaluating one of these functions. Generally that occurs because in your equation you supply values to a function that are out of its domain.
If you want to see what the internal representation of the equation in Netica looks like, you can generate a report by choosing Report → Horizontal Format from the menu and then Report → Equation.
Some example internal functions are:
_not _discretize _find0
_levels _select0 _Bernoulli