Old browsers = Evil Incarnate
I'm ashamed to say I spent at least two hours last week trying to fix a problem one of our customers was having with their layout. See my beautiful paint creation below to get an idea of what they were seeing.
My main problem was that I was using Remote Desktop to connect to the web server the layout was stored on, and it took me a long time to realize that the web server only had IE6 installed on it, which was why I saw the problem when I viewed the website from the web server, but it looked fine when I used (newer) browsers on my computer.
Once I realized it was contained to IE6 I got this scary feeling like I had seen this type of problem before. All it took was one quick Google search of "IE6 CSS bugs" and I found what I was looking for. Many things to the CSS Tricks website, who have this wonderful article about the major bugs in IE6 that tend to plague web developers. The bug my customer's layout had was the one he refers to as "The Box Model" bug. All I did was make the wrapper a few pixels wider and Wah-Lah! problem solved. Which is depressing considering how long it took me. :(