mistaken about something—is used of smth. concerning which one makes a mistake:
- I was mistaken about her age. I thought she was over fifty.
mistaken in something—
1. is used of an erroneous statement, supposition, etc.:
- The ancients were mistaken in their belief that the earth was flat.
2. = mistaken about something:
- “The papers have been greatly mistaken in my age,” said the divorced wife of the late Samuel Breakwell to a reporter.