Well, I don't know if you read Season 8 so I won't spoil it for you, but in that case Buffy has something something monumentally wrong, which is having dire consequences on her and her friends.
As for Buffy ever being wrong....yes, I think she has. Her intentions are always in the right place, and yes, she does have good instincts, but she's not perfect. She makes bad calls. I think a good example is Empty Places, where she orders the Slayerettes to go find Caleb. Clearly, they weren't ready for that. She didn't deserve to be kicked out, but that's another discussion.