A common pitfall of a CSS based columns layout is that the columns do not share a common height. Unlike a table based layout where the height of the table itself dictates the height of all of its columns, CSS columns are independent of one another in that respect. Now, this can be problematic (from a design standpoint) when you wish to style one of your CSS columns in a way that should extend all the way down to the end of the layout, such as giving a side column an explicit background color, a surrounding border etc.