if you are mistaken, you are wrong about something that you thought you knew or saw:
It can't have been my car. You must be mistaken.
I thought he said 12 o'clock, but I might have been mistaken.
We bought the rug in Turkey, if I'm not mistaken.
a mistaken belief etc is not correct:
Marijuana has few withdrawal effects, and this has given rise to the mistaken belief that it is not addictive.