It has been proven that if two negotiators begin to mirror each other's behaviour after only a few minutes, the conditions for an open, constructive exchange are very good and solid.
Instead of feeling ashamed or stupid when you and your counterpart copy each other's behaviour, you should congratulate yourself. Catching or mirroring body language is a sign that both of you are eager to build a relationship, connect and create common ground, even if you are not aware how or when the mirroring began.
Mirroring not only seems to make us feel comfortable in the presence of those people who behave in the same way, but also makes us believe them more.
With all these positive influences, you should still be aware that well-trained negotiators who are aware of the power of correct mirroring might try to influence you with this tactic. Making them believe that they are in a good, safe position can leave you vulnerable.