I guess we try, but therapists are human and prone to error. I have met therapists who seemed to just go with the flow and then, when they later explained what they did, it became apparent that every sentence actually had a purpose. With others it seems to be more of an attitude: know the general rules, understand the client's needs, be present and pay attention ... It can be very challenging to pay attention to every single word and gesture for an entire hour (or even 50 minutes), especially so if it is a long-term therapy. Which is why I think the therapist's personal therapy is so important. To err is human. I think it's actually an important moment if the therapist says that one word or phrase that didn't come out right and then listens to the client's reaction, and owns up to his/her mistake. Then that moment of doing or saying less-than-the-right-thing can be turned into something therapeutic.