Not having a residuary clause.

Kiplinger’s has identified the 10 most common estate planning mistakes:

Number 8 on their list is not having a residuary clause.

A residuary clause deals with everything you didn’t specifically name in your will, forgot to put in your will/trust/etc., things you don’t yet own but will before your death, and things you might not know you own. This happens more than you think! My family went to sell a property and found out there was a 4-foot by 25-foot strip of land as a part of it that wasn’t ours. When we asked the owner to sell it, he never even knew he owned it.

I often see this mistake on hold-made or computer generated last wills.