It's amazing how common it is to see something that doesn't make sense in the data and instead of deciding to investigate, they just move on. I'm guilty too sometimes. But almost every time I dig, I find something that invalidates it. Maybe an ETL somewhere is putting an empty string instead of NULL in particular cases. Maybe my join is duplicating certain rows. Maybe you got a table (or spreadsheet) that doesn't make sense. It all violates the assumptions of an analysis, but collectively we just say "can't eliminate all bugs." I hope the future is better.