About 20 years ago, I worked at the help desk of a software company. One day, a colleague took a call from an employee of a customer. That person thought he was being pranked by his colleagues, because every time he logged into our application, he was greeted with a message that he was a dick, and he wanted to know how he could undo it.
After some panic on our side, and asking around the company, it turned out it was an Easter egg one of our developers had created after he had become annoyed with (or by) one of our software testers, and entirely forgot to remove it before release. Unfortunately, the employee of our customer had the same login name, triggering the condition for showing the message.
The customer could laugh about it, and the developer got a stern talk from one of the owners, and I believe he had to audit our software for similar Easter eggs...