The old + in Outlook and the disappearing emails. Oh how many times I have told the user to click on the plus sign. But around 3 years ago I managed to persuade a client to move away from a 2Gb web hosted imap to an onprem exchange (internal reasons, don't need downvotes because it's not on someone elses server). Anyway, bucket loads of space, 60 users, 50Gb mailboxes so each staffs email archives were all pulled in to the live mailbox. At least once a month I get a call from someone asking where the archive is as they are looking for an email from year dot. I always tell them to actually search in outlook as everything is there.