I've mentioned this example before, but it's completely appropos:

My boss used to work for a call center. Every so often the entire system would crash for no apparent reason. After some detective work, they isolated it to one specific employee.

The root cause of the problem was the guy was clicking on a button in the GUI just to see how fast he could make the colors change back and forth.

The moral: You can NEVER be 100% certain how your software will be used now and forevermore.

And these days software doesn't run in isolation. You can test your software until you're blue in the face, get it working perfectly, then the company (ahem) that writes the OS changes something that causes your code to break.