Using Gmail, I have come across some emails that kill Internet Explorer if I open them. The emails that do it for me are ones from financial institutions, but I've heard that others have the same issue with emails from different origins. What happens is that when you open the 'bad' message, IE pops up it's "Security Information" box and says: "This page contains both secure and nonsecure items. Do you want to display the nonsecure items?". It doesn't matter if you select Yes or No, because IE will give an error message: "Internet Explorer cannot open the Internet site https://mail.google.com/mail/. Operation aborted" Then once you click OK, you get either an "Action canceled" or "The page cannot be displayed" errors.
This seems to happen only when using gmail via SSL: https://mail.google.com/mail You can view the message without any problems via gmail and standard http - http://mail.google.com/mail The problem only seems to happen in IE; using https://mail.google.com/mail via FireFox, I have no problems opening the emails. Another way around the issue is to open the email in a new window: turn on the keyboard shortcuts for gmail and then do a shift-o to open the message in a new window; it will open just fine that way. Sometimes just trying to open the message after a few times will get it to open successfully. Another thing that works is to open the message first using the standard http interface: http://mail.google.com/mail, then without closing the browser, change the URL to secure http - https://mail.google.com/mail then view the message without a problem.
The above was tested using fully patched IE6 on Windows XP, SP2 and using FireFox 1.5. It may act the same way with different versions of both IE and firefox.