1. Jointly own large bank or brokerage accounts.
2. Leave assets directly to a minor child or incapacitated person.
3. Forget to fund your living trust.
4. Name your estate as the beneficiary of your IRA or other retirement account.
5. Forget to have your plan reviewed every few years.
6. Forget to coordinate your beneficiary designations with your plan
7. Lose your original documents (it happens more often than you would think).
8. Rely on non-experts for advice.
9. Underestimate the value of using an estate planning specialist.
10. Use the ostrich approach to estate planning. (This is the most common method of planning/lack thereof!)
The reasons for most of these is obvious, others maybe not so much. The best thing you can do to learn more is to consult with an estate planning attorney.