The context: You're designing a multi-column layout with CSS using float. The frustration: There is a mysterious, tiny gap between columns or content when viewed in Internet Expolorer for Windows, versions 5 and 6. You try everything. Nothing works.
Upon further investigation, you find that the mysterious number of pixels added is exactly "3". There is hope.
If you hadn't already found it, Holly Bergevin and Big John not only document this bug (thank you, thank you), but also offer a workaround. (One that I've had mixed results with. Sometimes works like a charm--other times, when more complex styles are added, the gap still remains an issue. But just knowing of the problem can at least offer some solace.)
Over the past year or so, I've run into this problem numerous times, luckily stumbling onto the aforementioned writeup at just the right time. For those first trying to discover what's causing it--it can be nightmarish and impossible to try and solve. So, spreading the word on this bug can only be beneficial.
If you're struggling with tiny gaps or seemingly inconsistent width and spacing issues in IE/Win only--be sure to read about this one. Even if the problem turns out to be something else, the 3px gap could easily be the culprit in the future.