Many people know exactly what their limitations are. On the other hand, there are people who have few limitations, but the people around them (being of many limitations, themselves) often can't believe someone else isn't as hindered by the same kind of limitations that they are. As a result, the people who know what they're capable of, and don't have those same limitations, are faced with others thinking they don't know their limitations.
I once had a friend who was a mother, and she often talked about how she wanted her children to know their limitations, and "not have unrealistic expectations."
"How sad," I would think when she'd say that. Rather than encourage her children to try to overcome whatever limitations they ran into, this lady wanted to make good and sure her kids "knew" that they should never hope to achieve more than deemed they were able to achieve.
There's a certain talk radio host who frequently says how many people there are who will tell you that you can't do something, just because they, themselves, haven't been able to do it. This individual also frequently says, "Don't listen to those people who tell you you can't do something. Listen to the people who have done it themselves."
The world is full of people who have far, far, fewer limitations than a lot of the people around them tend to think they do.