I received an e-mail from a friend this morning who pointed out that, in Safari, pages on my site are "cut off" near the bottom. When the content section is shorter than the menu section (e.g., on some individual archive pages), the menu is cut off. When the content is longer than the menu section, the last three or four lines of the content is cut off. For both of these cases, the browser window must be shorter than the length of the menu. While I do use Safari 100% of the time, I don't make it a habit of looking at my archived posts or, as it seems, any of the other pages where the content might be shorter than the menu, which explains why I never noticed this error. It won't happen again.
I fixed it by applying the clear: both property right before I closed the container div.
Oddly enough, I couldn't reproduce the error in IE5/Mac, Opera6/Mac, Mozilla1.4/Mac, Netscape7.1/Mac, IE6/Win, or Opera7/Win — only Safari.