When I was in college, I wrote a simple worm to display a new year greeting on all computers it infects. Once it infects a computer, it did the following: 1. it replicated itself to as many computers as possible 2. Displayed the greeting (till user acknowledges it through a key press) 3. self delete (in the hope that it will quickly die by itself)
I seeded it in one of the computers in our college network. I didn't expect it to be so effective; It spread itself very quickly in the entire network. With self-delete, I thought it would die on its own. I was wrong. Machines kept infecting each other in a perpetual loop. The only way I could stop it was to write a new version that replicated, and cleaned the first version. This new version kept replicating in the network even after a year. This new version was not doing anything visible to the user, and I was saved :)