mistake   

Function: noun

1 : a misunderstanding of the meaning or implication of something <it is a mistake to think that the supreme or legislative power of a commonwealth can do what it will -- John Locke> <it is a great mistake to think that the bare scientific idea is the required invention -- A.N.Whitehead>

2 : a wrong action or statement proceeding from faulty judgment, inadequate knowledge, or inattention : an unintentional error <it would be a mistake, however, to drain all bogs -- Boy Scout Handbook> <gave him a ten-dollar bill in mistake for a one>

3 law : an erroneous belief : a state of mind not in accordance with the facts