Of course a T is allowed to make a mistake. It's impossible for anyone to be perfect and if my T never made a mistake, she'd intimidate me. Just like with everyone else, what matters is how big and major the mistake is. A mistake is showing up for an appointment a few minutes late once or tripping on her words and saying something she doesn't completely mean but then later correcting herself. Abusing a client in any way including violating boundaries is not a mistake. The T knows better but did it anyway.