A significant challenge to deploying mission- and safety-critical autonomous systems is the difficulty of creating a credible assurance argument. This paper collects lessons learned from having observed both credible and faulty assurance argumentation attempts, with a primary emphasis on autonomous ground vehicle safety cases. Various common argumentation approaches are described, including conformance to a non-autonomy safety standard, proven in use, field testing, simulation, and formal verification. Of particular note are argumentation faults and anti-patterns that have shown up in numerous safety cases that we have encountered. These observations can help both designers and auditors detect common mistakes in safety argumentation for autonomous systems.