There are no insidious little details that will trip you up because you haven't gotten them exactly right. The only errors you can make are really big ones, like not listening to the client. Saying the wrong word at the wrong time won't make much difference. Not listening to the client would make a big difference.
I am a bit reluctant to even write up a list of things you can do wrong. You are much better off thinking about what you would like to accomplish. As long as you get the client moved in the right direction you are doing fine. It is the results that count, not how many course adjustments you made along the way. However, it can be useful to be conversant with the ways of doing things wrong, so that you can recognize when you do them and quickly do something else.
Really the only thing you can do wrong is to not adjust to what is going on. It is no big deal if you make a "mistake". But if you don't notice what the result was and adjust for that, that is when you are going off the track. Sometimes doing the "wrong thing" can turn out to be useful. If you notice the feedback you get from the client it can provide an insight into changing things.
Have your attention on the feedback you get from the client, not on whether you are doing things correctly or not.
Here is a quick list of things you probably would avoid doing to clients. You can try doing them in training to see how they feel and what happens. But don't go around spending much attention on them.
- coming in late
- being rude and unfriendly
- looking or smelling repulsively
- not looking at the client
- not listening
- deciding what to do without any perception of the client
- talking so you can't be heard
- not acknowledging that the client speaks
- misunderstanding what is communicated
- trying to impress the client
- trying to seduce the client
- touching the client in unacceptable ways
- bringing up subjects you don't intend to help her with
- talking about stuff without relation to the client
- changing from subject to subject without handling anything
- overwhelming the client
- distracting the client from the process
- backing out when things start to change
- giving robotic questions or directions
- continuing a technique when nothing is happening
- cutting the person's communication
- finishing her sentences
- using false friendliness
- lying to the client
- letting the client control the session
- poking into the person's face, so as to make her space smaller
- spreading around the secrets revealed in session
- leaving the session without explanation
- scolding the client for the way she is behaving in session
- interpreting the client's answers for her
- telling the client that there are certain things she shouldn't say in session
Some of these reflect norms for social conduct that are fairly obvious. Of course you wouldn't expect to come in drunk to a session. It is more likely that one would miss out on some of the basic session skills as a new facilitator. You might accidentally distract the client from the process with an otherwise well intended comment. You might change the subject without noticing it because you get confused. You might miss something the client said because you were busy looking at her.
As long as you have the basic intention to be there with the person in front of you, and help her to have more choice in life, then most actions follow quite logically. Where you might miss out is if you accidentally make some minor rule of processing more important than the overall purpose. You might be trying to execute the technique right, but then not noticing that the client is no longer there with you.
It is the big fundamentals that are important and that will make the difference. It isn't the little technical details. Get the universal basics into your blood and you will do well with any client under any circumstances.