Neil Hendershot has an excellent post about POA Abuse here which includes the research report "Power of Attorney Abuse: What States Can Do About It."

The most egregious abuse is where the agent simply takes (that is, steals) the principal’s money for is or her own purposes.  But there are many other type of abuse.  For example, one often sees agents under a power of attorney creating joint accounts in banks or brokerage houses.  This frustrates the intent of the principal that the assets pass under his or her will to intended beneficiaries.  Similarly, one sees agents changing beneficiary designations on IRA’s, life insurance policies, annuities, and other contractual benefits. 

Sadly, this is often in a family context where one child is acting to the detriment of his or her siblings.

The principal thinks that all the children are being treated equally because that is what the will says.  After the principal’s death, the facts emerge and siblings see that what passes under the will is relatively small compared to the assets that pass to the agent via beneficiary designations.

Our prior post  How to Act as an Agent under a Power of Attorney talks about how to stay out of trouble when acting as an attorney in fact,

For a discussion of power of attorney abuse as illustrated by the unfortunate case of Brooke Aster,  whose son Anthony Marshall was indicted for abusing her POA, see this NY Times article.