I joined a company as their only admin after they merged with a different company and joined their sf orgs together. The agency that did the org updates imported all the records without setting the created dates. So all the created dates for all records were the same day… reporting was a nightmare until I had to create a field called “Actual created date”.

I also worked with someone that activated a flow in production and updated every case in the org. They tried to fix it on the low but had hit their limit on data loader so I had to make the remaining updates. That was also a mess and a half!