Or, similarly, I set up an HTTPS proxy on my Mac, and set Wireshark to listen. Then, had someone else log in to a different account (say, the guest account) on the machine, and asked them to log into gmail, say, as usually via HTTPS (with a fake user/pass). A warning does pop up about "insecure connection", but most people just dismiss it and go ahead and log in - and wireshark intercepts username and password.

Classic MiTM, well known, but still freaky to observe how easy it is to set up.