I get caught on this in two ways all the time. First is when I go to check my webmail, start typing my name/password before the page is done loading, and then the browser (annoyingly) changes the focus back to the first textbox when it finished loading, showing at least part of my password if I'm not careful. Then I'm left with something like

Login: ivarnelissword
Password: **

This happens to me all the time in both Internet Explorer and Mozilla, and it's gotten worse going from broadband back to a dial-up connection. The other way is when I'm using Internet Explorer for ftp. It'll let you login to an ftp server by typing a url like: ftp://ivarneli:password@ftp.server.com. Of course, then you're typing your password in plain view, so to be tricky, you just type in ftp://ivarneli@ftp.server.com and a dialog box comes up asking you for your password (and hiding it when you type it). But what happens when you press OK? You look at the URL box at the top of the window and see your password right there! It gets even better... IE saves it in the history, so the next person to use that computer can easily see your password and log in to your ftp server if you're not careful and clear the history.