I apologise if my implication was that only part-time instructors make errors but my firm belief is that we can become rusty very quickly when it comes to instructing. Correct patter is usually the first to fall by the wayside, followed by knowledge of the syllabus.
Seeing as you are obviously probing for some controversy Mr.VT - ask me who would I sooner recommend learning to fly with - someone who instructs at a flying school on days off from their full time job or, a full-time career instructor at the school.
VFE.