TEXT files!!
Our corporate banking app used to have this. I could browse onto a secured network share where the transfer files were stored. Only SysAdmins and certain Finance peeps had access. Within this folder I could see many text files that were all plain text which included account details that our Banking application would pull data from when making transfers to Suppliers.... Some transfers could be in the millions. I still can't believe it today that it worked like this. Thankfully the software was upgraded to an online version so the server was decommed.