I once built a webapp to help players in a game I played a lot at the time, mostly to assist myself and my clan in finding good combinations of equipment by displaying the stats in a clean and sortable format. The actual web frontend was fairly simple to create, but the data parsing side of things was an absolute nightmare due to the dataset being an absolute mess. In the end though, it proved helpful in learning how to handle different edge cases when processing massive datasets you have no control over.