Why must software handle errors in addition to avoiding bugs?

"Even if code contains no bugs, it is still susceptible to errors. For example, a program could connect to a remote database to query and process data. However, if the network connection is temporarily unavailable, the program will not be able to execute properly. Because of the potential of such errors, it is important to write code that is not only bug free but is also robust and resilient. We must anticipate possible error conditions and write code to detect, prevent, or recover from such errors. Generally, this is referred to as error handling . Much of what we now consider Software Engineering was pioneered by people like Margaret Hamilton who was the lead Apollo flight software designer at NASA. During the Apollo 11 Moon landing (1969), an error in one system caused the lander’s computer to become overworked with data. However, because the system was designed with a robust architecture, it could detect and handle such situations by prioritizing more important tasks (those related to landing) over lower priority tasks. The resilience that was built into the system is credited with its success [11]."