In my first few months as programmer in the 90's, I realized that human inputs were sketchy. It was from a form field for which state someone was from (USA). Fifty possible states, 2500 different entries. Sure there was a bit of garbage, but 95+% were recognizable states...but why random capital letters? How do you get a space in Wyoming?

It was a good lesson at the start of my career that I see playing out over and over. When I see some cool demo of statistics across the country or globe, I'm more impressed by the effort of cleaning the data than the stats behind it.