[AKluge]

One of my favorites was to open the frame buffer on a Unix workstation, then logout leaving a background job running.

A little after someone logged in, the code would do something cute, like draw a crack on the screen. I once saw the guy next to me back away from the computer, and ask, "It's not supposed to be doing that is it?"

Perhaps not quite black hat, but I was amused at least. :)

[ASalazarMX]

Once I worked at a bank developing software for IBM is (AS/400 then). As any other programmer, I slowly built a toolset to make my work easier. My boss saw one of them and asked me to clean it up for the operators to use, as it made their work much easier. I did, they used it daily, everybody was happy.

Months later my boss called me at 3am, very stressed because the operators told her there was a skull in the screen. Fearing the worst, they traced the program back to me, and I sleepily explained: "the skull appears on Fridays the 13th, it's a visual Easter egg, the program works the same as any day".

In hindsight, maybe it wasn't the smartest thing to do. I was lucky my boss and the others were very tolerant, but not removing that Easter egg could have gone very wrong. AFAIK they didn't change it, seems like they liked the predictability. It would be perfect to scare newcomers.