(1) Not understanding the law and what you are entitled to before agreeing to a Settlement agreement. For example, a lot of potential payors on child support or maintenance don't realize how much the law and the legal landscape has changed in the past decade, and therefore they end up paying far too much, because of their preconceived notions and expectations about what they were going to have to pay (notions which are probably based on outdated societal expectations). Recently, higher credits for noncustodial parents were allowed, but you have to know how to ask for them and why. Also: a lot of spouses leave huge assets on the table that they are legally entitled to have. This happens commonly with retirement accounts but also happens with other assets.
(2) Not knowing how to collect the assets that you are awarded in the divorce case. This is a problem with child support. This is also a problem very commonly with retirement accounts. Litigants don't understand that collecting these does not automatically happen when the court enters their orders. More paperwork is many times required especially in the two above-mentioned situations.
As an attorney, I know that some people will need to do some of the legwork on their own, but a great time to consult an attorney is when you have been offered Settlement Documents by the opposing side. An attorney can charge you just for the few hours to review the document and consult with you about your rights. This is well worth the expense for any litigant. Additionally, if that attorney knows your final Settlement agreement, she can advise you on what you will need to do to collect your assets. Something easily defined like the required order to get to retirement assets or the required request for child support payments directly from noncustodial parent's workplace can also be accomplished on a "flat fee", which means that you will know how expensive the work is before the attorney does it. This is not a viable option with an attorney for a full dissolution proceeding because of the unpredictability of the work needed, but it is an option at these very specific points in the case.