How does a mistake like this happen in an era of GPS? Just wondering.
There are actually two questions here. How does it happen and why does it happen.
How it happens is easy. Distraction, complacency, lack of situational awareness. All those topics they teach us in CRM that go out the window on a fine weather day.
Why it happens is harder to answer because it begins to ask other questions that not all pilots and air operators are comfortable answering.