Star Trek: 10 Best Holodeck Episodes
8. The Next Generation: Elementary, Dear Data
Data was an incredible being to say the least, so challenging him often proved be difficult. Geordi LaForge ran into this when he tried to put together a Sherlock Holmes themed mystery program for him to solve. The problem was that Data had every Holmes story and research on the character in his memory. It was easy for him to solve the program.
Dr. Pulaski overheard the discussion and stated that she did not believe that Data was capable of solving a mystery that was 100% new and unfamiliar to him. Believing his friend was up to the challenge, Geordi instructed the computer to create a unique Sherlock Holmes mystery with a villain that might actually be able to defeat Data.
The unintended consequence was a holographic version of Professor Moriarty who was self aware, knew he was in a simulation and could access the Enterprise. Moriarty could actually call up a computer access terminal and utilize it. He was even able to cause damage to the Enterprise. While they did get the program into storage, Moriarty was eventually released and caused more problems down the road.
The Enterprise-D had more than a few enemies, but having the holographic rendition of one of literature's greatest villains as one of your nemesis certainly stands out.