Some of the things I've personally encountered are attempts to service refrigerant without first weighing tank, and a claim that R-22 systems can no longer be serviced due to EPA refrigerant ban; I suspect these are very common grifts.
Still worse, a brazen attempt to service an older R-22 system with R-410A, which would have completely destroyed a heat pump that I ended up getting an additional 5 years of serviceable life out of (after dismissing that clown on the spot).
Other ridiculous mistakes I've had to deal with are incorrect wiring of air handler emergency heat source during initial installation that prevented system from cooling; and on that same heat pump system less than a year later, discovering an improperly secured lug within exterior disconnect box that eventually created enough heat to fry ~6 inches of insulation before failing open and killing power to compressor (in retrospect, city inspector not only should have caught this, but should have required the contractor to replace the disconnect box altogether to remain in compliance with prevailing residential building code).
The grift has gotten so bad over the years that a few friends---a professional ME and electrician---have gone out of their way to earn § 608 tech certs[1] to legally purchase refrigerant and have sufficiently tooled themselves up to handle common failure modes.
[1] https://www.epa.gov/section608/section-608-technician-certif...