Mistaken, incorrect.
Example:
The chess wizard's parents formed an erroneous idea of his intelligence because he didn't talk until he was six.
Erroneous seems to be used most often with words that suggest mental activity. "Erroneous assumptions" and "erroneous ideas" are two very common phrases in English, perhaps because we suffer from so many of them. "Erroneous information" is also very common, and it leads to erroneous decisions, erroneous theories, and erroneous conclusions.