Be mistaken

[Mr.Lucky_One]

"He may be mistaken about what he has seen."

I have some difficulties with getting the sentence above.
What does "to be mistaken" mean? Is it passive voice or what?

[Raymott]

"He may be mistaken about what he has seen."

I have some difficulties with getting the sentence above.
What does "to be mistaken" mean? Is it passive voice or what?

No, it's an adjective from the past participle. If he has made a mistake, he is mistaken.
However, this can be a passive construction. "He may be mistaken for a robber if he wears a mask" = "Someone may mistake him for a robber if he wears a mask".
Another example:
"I'm mistaken" means "I'm wrong".
"I'm mistaken for an alien" means "Someone has mistaken me for an alien."
The context should signal whether it's a passive construction or an adjective.