I once had two gmail addresses forwarding to each other on accident. If you're wondering if Google has some sort of check for that, the answer, at least at the time, was several million nos.
I stupidly tried to do that some time ago (got confused juggling with my spammed mail addresses that are all forwarding to my main email address), and Google told me it was a bad idea.
A student group I was a sysadmin for had partially switched to Google Apps for mail from a self-hosted mail server, but for those who were still on the local server, some of them had set up their email to simply forward to their university address. University addresses expire an undetermined time after you graduate, so eventually when this happened to someone who had setup forwarding, it would trigger a horrendous forwarding loop of bounce notifications which would forward back and forth between our server and the university server and rather quickly fill up our (fairly small) mail partition and crash the whole machine. Thankfully these days everyone is on Google Apps so if they're doing forwarding they're doing it through that and thus I don't think you get bounce loops, and most people just check that address directly anyway.
