Last week I got an invitation for GMail. I clicked the link to create my account and to my surprise it gave me an alert when I tried creating a GMail account about the enabled Popup blocker(It opens a new window for singning a new account). Whenever there is an error in sending a mail or performing some action on my mailbox fails, it immediately gives a message alert.
I had this experience with MSN WebMessenger too. It has popup blocker detector that detected my enabled Popup Swatter and informed me about that.