This is an example of what you should not do if you want another job. I (stupidly) once put some code in a program I wrote that said “Keith has not been paid”. It kicked off two months after I finished a project. I was suspicious that I would not be paid, and I wasn’t. It was a stupid thing to do because it guaranteed that I would not be paid ever, and it burned some bridges that may have eventually proved lucrative. It hurt me, did not hurt the target, and was a real adolescent idea. It is up there with the top ten stupidest things that I have ever done.
Later on I told a fellow programmer about it. Rather than take it as a cautionary tale, he put destructive code in all of his programs. If he was ever fired, his code would wipe the database a month later. A supervisor discovered the code, the programmer was marched to the door, and I had to review tens of thousands of lines of his code searching for this idiots trojans.
The programmer at Fannie Mae did not write a complicated worm or anything. He changed a supervisor script that had access to several thousand servers. The script checked the date and when the time was right it would have killed a whole lot of data on all of the servers. It was just a few lines of extra code.
The kick is, that the guy screwed up this stupidly simple three or four lines of code and the reason he was discovered is that the script crashed the first time anyone tried to run it. Access logs pointed to who had made the change. The guy was a bad programmer and deserved to be fired. Now he faces 10 years in jail.
The moral of the story is, as a programmer, you are in a position of trust. Management has no idea what it is you do and how much they depend on your honesty. Ethically, you have to act in the best interests of your employers. If you betray that trust, there are always people who are better coders than you, who can 1) track you down, and 2) fix any damage in a short time. The best revenge is to get another job that pays even better than the one you were fired from.