Power of Attorney? If you sign one giving another person permission to act for you can they use it legally to?

sign away your parental rights to your children and then adopt themself?
Answers:
No.
A power of attorney does not remove YOUR rights to act in your own name, and does not allow the attorney-in-fact (the person you have the authority to) to contradict you.
You can override any action taken by someone with a POA if it is outside the scope of the POA.

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Not usually.
But... If you are planning to sign a power of attorney - maybe you should consult with an actual attorney before signing anything.
Sounds like you need a "limited" power of attorney which specifically states which act the bearer of the document is allowed to do.

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No. Having Power of Attorney does not grant that person rights to make any decisions that benefit themselves. For example, if I have power of attorney for you, I can make financial transactions on your behalf, I can't however make financial transactions giving myself your money.
That being said, if you are worried about someone trying what you are asking then they are not the right person to grant power of attorney to.

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Not usually.
In most cases it only gives them the power to administer over business and financial matters.

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I doubt it, I feel it's a misuse of the power.