Root cause of software bug

The user executed untested code. Because of time constraints, it’s not uncommon for developers to release untested code—code
in which users can stumble across bugs.

The order in which statements were executed in actual use differed from that during testing. This order can determine whether software works or fails.

The user applied a combination of untested input values. The possible input
combinations that thousands of users can make across a given software interface are simply too numerous for testers to apply them all. Testers must make tough decisions about which inputs to test, and sometimes we make the wrong decisions.
The user's operating environment was never tested. We might have known about the environment but had no time to test it. Perhaps we did not (or could not) replicate the user’s combination of hardware, peripherals, operating system, and applications in our testing lab. For example, although companies that write networking software are unlikely to create a thousand-node network in
their testing lab, users can—and do— create such networks.