If you use "Nice menu" module and put a "nice menu" block in the left or right sidebar, the fly out will not show because of the css overflow:hidden. How to fix it, and why is it necessary?

Overflow:hidden css code protects your website from been breaked by the users. How it works: If the content is too big for the column, it will be hidden, and it won't make the column width bigger, than it is supposed to be.

Example: if user writes a very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very_very
long word, nothing will happen to your columns, the part of the content will simply be hidden, and won't make the right column go below the center column, because of a very long content of the center column.

If you want to use "Nice menu" module you should just find Overflow:hidden parameter in style.css and remove it.