What makes a bad conception of self? Two possibilities come immediately to the fore.
First, a conception of self might prove wanting in that that it fails to correspond to or make adequate sense of the object which it sets out to explain. If a self were divided into rational, spirited and appetitive parts, set against one another, this model could well maintain logical relations between the different parts and exhibit internal coherence without thereby providing the explanatory power hoped for. Be this due to dubious empirical bases or fanciful imagination, the result is the same: this is not the best available account. Here, the explanation is sufficiently robust but of the wrong type.
Second, a conception of self might fail in that presupposes too much of and overextends commonly accepted notions of self. Consider that a liberal society attributes to the subject, here a stand-in for self, inalienable rights to life, liberty and property. Such a view holds up under scrutiny, but such cannot be maintained for its seemingly reasonable extension in the view that these rights are essential properties of human nature and selfhood. In short, a relatively uncontentious position can lead to the overly metaphysical view there can be no self without rights. Here, the explanation is of the right type but is overly robust.
If the possibilities given above suggest something like a sliding scale between scope and strength, there is as of yet no reason to suppose that these exhaust the measures of bad conceptions of self. Indeed, it remains possible that a considerable list of such illusions, mistakes and misinterpretations could be compiled so as to provide much in the way of instruction by negative example.