It had huge unintended consequences, but Robert Morris actually just wanted to know how big the Internet really was when he launched the so-called "Morris Worm." Robert Morris was a student in Cornell when he wrote and launched the virus on November 2, 1988. Although he didn't have nefarious intentions with the virus, he made a critical mistake in the code. The result was that the worm crashed about 10% of the computers plugged into the Internet.

 The worm took advantage of a number of security vulnerabilities in order to infiltrate the computers. Once it had done that, it would query the computer to see if there was a copy of it already running on that particular computer. If the computer said yes, then the worm would stop and not run again. At least that's how it was supposed to work. 

That was his big mistake. If anyone realized that the worm existed, they could've easily programmed their computer to answer YES to the worm's question, and stopped it that way. To make it harder, Morris programmed the worm to run even if the answer was yes, 1 out of 7 times.  As a result, the worm started running multiple times on many computers, and brought down about 10% of the computers running the Internet, because there were too many copies of it running. 

As a result, Morris was the first person to ever be convicted under the United States Computer Fraud and Abuse Act, and sentenced to 3 years of probation, 400 hours of community service and a $10,000 fine. 

(Source)