I have my Mac (running Tiger) use both British English and Spanish keyboard layouts. My passwords use various symbols as well as letters and numbers. Several times I have had passwords rejected, even though I know I typed them right. This is due to the differences in the keyboard layouts, and sometimes the OS seems to switch layouts at random.
While this is not a problem normally -- I can just use the menubar item to switch back -- it has happened a few times when I have been at the password entry box for a locked screensaver, from where the menubar isn't reachable.
I remembered that sometimes the input source menu item would hijack my Quicksilver shortcut (the system uses Command-Space for this by default). So I tried hitting Command-Space in the password dialog, and then entered my password. It worked! And even if you are running Quicksilver.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060329074019948