I started mystery shopping years ago when I had just moved to a new city I wasn't familiar with, so I didn't know where anything was, and online maps and GPS weren't anything like what they are today. I spend hours driving around a strange city to do a few $10-15 shops.
I decided mystery shopping was a waste of time and gave up after a few months. I didn't come back to it for a long time, and by then I had figured out at least a couple of the stupid things I had been doing. I'm sure there are other idiotic mistakes I have yet to make.