Absolutely. A will's validity is determined after death in probate court, which is often long after it was executed. Disappointed beneficiaries often try to challenge the will's validity. Frequently, family members will attack the deceased's mental capacity at the time it was executed. It's common because Probate essentially sends an invitation to people to challenge a will, and provides a convenient venue as well. This is just one very important reason why you want to avoid using a will to articulate your postmortem wishes and to avoid probate entirely.
Wills and Living Wills Lawyer