It is very easy to make mistakes with your estate plan that are not discovered until long after it is time to administer your estate. Most estate plans actually do very little while you are alive. They tend to sit there so they are ready after you pass away. For this reason, mistakes are not discovered until it is too late to do anything about them.
A common mistake is creating a Trust and not making sure that an appropriate person is named as the Trustee. Many people choose to appoint a family member and long after the fact it is discovered that the family member appointed is not very good at actually administering the Trust.
You can guard against this possible mistake in a couple of different ways. First, review your Trust from time to time and make sure that the person you designated to be the Trustee is still an appropriate person. For example, if the designated Trustee has incurred significant debts, you might want to designate someone else. The second way you can guard against this mistake is not to appoint a family member at all. Instead you can designate a professional Trust administrator.