Random folk practice that drove me nuts, Outlook "Deleted Items Folder" as storage, never actually deleted, ever.

20+ Gigabyte mailboxes, which were actually a threaded, searchable, rich content database. (Exchange's superpower (when it works) is a almost perfect online/offline sync able database -- You can send an email in your offline client, when you go online it'll get sent, your calendar updates will sync, etc)