At a company I worked at about 15 years ago, they dismissed (let go/fired/etc) one of the employees who also happened to handle much of the computing environment.
The president and vice president (and I believe at least one corporate lawyer) asked him into the conference room where they told him that he was fired and not allowed to return to his office. He said "may I make a phone call?". They said sure. He picked up the phone, dialed a number and said "Implement Plan B" and hung up the phone.
The pres and vice pres and lawyer went totally ape-shit. The place went into instant lockdown and we were pretty much unable to use the corporate network for about a week while they went through everything, changed all codes... musta cost a small fortune in both time and money.
A couple of years later I ran into the guy who was fired and I said "wow, do you realize what happened after you said those words? BTW, what exactly was plan B?" He said "I called my wife and said "implement plan b" but she had no idea what that meant, I was just joking".
Wonderful payback to some really stupid employers (they totally deserved it). Totally made my day.