I recently took on a new client that had some 16ch D1 Everfocus DVRs with 500GB hard drives that he was told would record for a year. He was only getting ~7 days of retention. He only found out after having a break in and couldn't figure out how to recover the event. We added a 4TB drive to each recorder to ensure at least 30 days of retention.
I also had a client that we installed some panoramic cameras for the first time and we had underestimated the storage needs because the VMS calculator didn't have an option for that specific camera. We ended up simply adding another drive to cover the miscalculation, but that is an easy fix in the grand scheme of things.