Today I was trying to troubleshoot a design tool I created that worked perfectly in FireFox but not in Internet Explorer. There was an element on the page, a <div>, that IE would not apply the CSS style to if the style was applied based on the id attribute of the <div>. I could however apply styles to it if I added a class attribute to it and wrote the class in my CSS stylesheet.
It made no sense. I'd assigned styles based on the id attribute all the time, and had never seen this problem in any modern browser. What the heck was going on?
Finally I saw my mistake: I had left off the closing bracket of the <div> that was the parent of the <div> I was having trouble with. FireFox was "kind" (aka presumptive) enough to close that tag at runtime, and IE was not.
Had I trusted my experience a little quicker, I'd have looked at the HTML code for structural problems first before trying to figure out a way around the problem.