I’m not sure if it was the biggest but non-representative data has probably caused the biggest headaches overall. This is usually due to some level of legal paranoia - some legitimate, some possibly not.
The first time this happened was at the BBC about 13 years ago where they had a pretty extreme level of anxiety about us seeing production data due to some law to do with children’s data. It’s also happening at my current company due to the sensitivity of client data.
In some companies you can simply do a production database dump and restore it to staging and you’re done. This is an incredible and underrated boon to productivity when you can do it - not just for staging but also for dev environments.
In most of these other places I would have liked to have built a demo data pipeline that engaged in some degree of sanitization, deletion and anonymization and spat out a minified but still representative database dump every morning but it almost always fell by the wayside as most developer experience projects tend to.