I once released a spamming tool on /b/ that zeroed out the user's hard disk except for the boot sector, into which was installed some code which displayed a duckroll upon starting the computer, and hanging.
Tell me your dangerous programs you've written, /prog/.
I once wrote a program that would check the hard drive for copies of the GPL and try to use subliminal messages to try to turn the user into a pedophile if it found any. Later I took the subliminal message code, obfuscated it, and contributed it to a reasonably popular GPL project. It's been several years, and they're still using that code, unchanged, and no one has even questioned the strange way that it's written because the entire project is such a mess.