Please don't banter here.
Specialty subjects can be great to teach- and quite lucrative as well. However, in many areas you can't really fake it...the gig has to matched with a teacher who actually has at least some actual knowledge of the field. It reaches a point where no textbooks are likely to EVER exist for the specialty, and simply looking up definitions isn't going to qualify one to function in advanced business, technical, and other areas. If a teacher can't be provided who knows what they're doing, one maybe should consider not accepting the contract...odious though that concept may be.
I've been very lucky; my life as an easily-bored Celtic dromomanic compulsive reader has finally found a home. An MBA and 2 B.Sc degrees in the sciences/maths, plus a pathologically diverse experiential background, mean I can teach an exceptionally broad palette of subjects. However, I still turn down some work simply because I have no knowledge of the subject. (For example, I just turned down a gig teaching Cost Accounting, which thank God I only know a little about...
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Another aspect of specialty subjects...it's tempting to try and chase down every field that people ask you to teach, and to provide a program to match all those fields. However, you quickly come to realize that it isn't worth it. Developing a course of study is hard, labor-intensive work...too much for something that you're probably only going to use once or twice. It's usually best to confine your efforts to fields that truly have a market and will make it worth your time and trouble.