Lean processes prevent human error in three basic ways. First, through the design of quality processes including testing for robustness, the creation of standardized work instructions with quality key points and the effective training of the people using a method at least as good as Job Instruction. Second, processes are error-proofed whenever possible. There are various types and degrees of error-proofing. Rather than list them, I will point you to John Grout's excellent work over the past decade in compiling these great examples of pokayoke (Japanese for error-proofing).