Good points. Unfortunately there are a lot of "hacked together" services out there. I can think of one site I had to use recently for event registration which

Good points. Unfortunately there are a lot of "hacked together" services out there. I can think of one site I had to use recently for event registration which suffered from the flaw I mentioned. (Note that I have no choice over which service to use here, as I don't run the event.) The site puts each event under different directories, not subdomains as it should. Firefox saved my login details for one past event and wanted to use them for another event. I don't know the extent by which the event pages can be customized, but if you could put the right JS on one event page then it seems there are multiple approaches to getting credentials and potentially obtaining sensitive information. Fortunately in this case an attacker wouldn't obtain much of value best I can tell.