This is really common sense but sometimes can be difficult in the moment. One thing you should NEVER do in meetings with customers or partners is show obvious frustration with members of your team. Even worse is starting to argue with them. It leaves a bad taste in everyone’s mouth.
Even if it is subtle everyone at the meeting will pick it up. You will have an extremely difficult time recovering from it and your customers or partners will leave the meeting with a really bad feeling about your company. Don’t be surprised if you never hear from them again or if the deal slows down and dies from that point.
It comes across to customers that there are major issues with your team (even if there aren’t) and creates a hostile and uncomfortable environment. If this happens the best thing to do to recover is ignore the frustrated party and divert the conversation back to what the customer cares about.