Years ago when I did freelance work writing software for custom phone systems, I had a shady client up in Marin. Suspecting they would not pay, I slipped a feat

Years ago when I did freelance work writing software for custom phone systems, I had a shady client up in Marin. Suspecting they would not pay, I slipped a feature into the system where if I did not dial in and punch in a secret code within 90 days, it would stop functioning and display a cryptic "database error". Sure enough, the client didn't pay, and right on schedule, I got a customer support call mentioning the database error. I told them "actually it's working perfectly, you didn't pay your bill". I later found out they went through several other developers and then went out of business. I am sympathetic to small businesses that have money problem, but some people are simply shady and must be avoided.