Employers who keep advertising for the same position because they want a perfect match for their requirements--not even a 90% match. I remember many years ago working for a startup where we interviewed a candidate who very nearly the experience we wanted for a firmware developer--but there was a belief that we could not afford to spend four or five months waiting for this guy to get the particular experience we needed with DMT. Nine months later, we again were sitting around a conference room, discussing a candidate for that same position, who was again, not exactly a match. I reminded everyone, "So, if we had hired [name now escapes me] nine months ago, he would have the experience we need now, wouldn't he?" And everyone looked sheepish.
If someone has the intelligence, but lacks some very specific experience, and you don't have an alternative available, does it really make sense to keep waiting, and waiting, and waiting for perfection to walk in the door? I don't think so.