A power of attorney (POA) is one of the most powerful (and potentially risky) documents one can sign: It gives a third party "agent" the ability to control the assets of the "principal" as if the agent were the principal. Depending on how broad the POA is, that could mean anything from controlling one's financial accounts to controlling everything: healthcare decisions, investments, property, and accounts.
With that much power comes a duty to act in the principal's best interest. As you might expect, that doesn't always happen. And if an agent is abusing his or her power, and the principal can't revoke the POA (a typical example would be a principal who is mentally incompetent), you might want to challenge that POA in court.
How? Here are a few ideas: