Although most users would never do such a thing, we just had two users hose their accounts because, for whatever reason, they setup Remote IMAP Retrieval & Remote SMTP under ACCOUNT SETTINGS > CONNECTIVITY > EMAIL RETRIEVAL and SMTP ACCOUNTS for the very same account they were on. This created a never-ending recursive loop resulting in 100,000 duplicate copies of every single message (of which there were originally 3,000) resulting in 300,000,000 messages before their accounts thankfully ran out of space. (And I was wondering why CPU and I/O were higher than normal!)
This shouldn't ever be a common issue (first time I've seen it happen in 10+ years), but if it is in anyway possible can an idiot safeguard be added to these functions so that they cannot be set to an account *AND* server that is exactly same as the account and server that they are already logged in as?
(I can see a legitimate use for Email Retrieval for the same account on a different server, such as keeping accounts on two servers synched while waiting for domain propagation to complete for a domain that is being migrated to or from your services, but they should have different hostnames...and I can see a legitimate use for Email Retrieval for the same hostname but for different accounts...but never a situation for both matching!)
Honestly there shouldn't be a way for a user to shoot themselves in the foot by setting it to retrieve from itself...because if a user can they eventually will.