An argument can be "decent" without being right. If you want an example, and can follow it Kurt Godel's ontological argument looks pretty decent. Consider that:

A) It is a logically valid argument

B) The premises sound fairly plausible (we can on the face of it imagine some sense of a "positive property" which would satisfy the premises)

C) It is not immediately obvious what is wrong with the premises

The wrongness can eventually be seen by carefully inspecting the premises, and checking which would go wrong in a null world (a possible world with no entities at all). Axiom 1 implies that if an impossible property is positive, then so is its negation (since an impossible property logically entails its negation). Axiom 2 says that can't be true - a property and its negation can't both be positive. So together these are a coded way of saying that all positive properties are possible properties. And then Axiom 5 (Neccessary existence is a positive property) goes wrong, because necessary existence is not a possible property in the null world. So it is not a positive property. Axiom 5 is inconsistent with Axioms 1 and 2.