I'm curious about this too, but kind of afraid to try it.
Yep, that's why I posted rather than trying it myself - and having to clear up any mess created.
Best answers that I can find is that modern email softwares will usually have various ways to spot and prevent any such misconfigured forwarding loops, Autoresponders responding to each other in a loop, etc.
(If for no other reason than to protect the providers own servers storage/bandwidth from potentially getting swamped in a kind of DDOS).
But it can still happen even though it's not as likely to happen it used to be.
Users will always do odd/unexpected things when changing configurations and mistakes (or malice) will happen..
However if such a loop does start up then it will 'break' when a mailbox reaches it's storage limit and then rejects any new emails being received, so that there is nothing new to forward or respond to..