According to the Center for Universal Design, headquartered in North Carolina State University, UD has seven basic principles.
Tolerance for Errors
The design has a high tolerance for mistakes. The idea behind this principle is safety precaution. It minimizes hazards and the adverse consequences of accidental or unintended actions. The design has a built in safety measures in terms of careless or forgetfulness.
In the picture a nail gun requires a to activate the safety button before pulling the trigger thereby removing the risk of accidents while the gun is pointed somewhere else.
See 6. Low Physical Effort.