Meetings in which clients are included deserve special attention. If you are new to business you may not appreciate how important clients are. Technical people especially - those engineers again - can think their technology or solution is God's gift and that only an idiot could fail to understand its value. Even if that is so, no one ever got very far by insulting or talking down to a client. You may have the brains but they have the money. Or maybe the client has a problem - real or perceived - that is somehow your responsibility. Now I'm not one to believe that the client is always right. When you are selling a product or a service, you may have to 'educate' the client as to how you will solve their problem. But I guarantee that if you are dismissive or condescending, you'll get nowhere. As a junior person, best to keep your mouth shut until you are confident that you can be deferential yet communicate your issues. Seek advice on how to handle client challenges.
Another mistake newbies can make is challenging or correcting other members of the company in front of a client. Suppose you are in a client meeting and your boss says "Sure, we can have that report for you in a week." Now, you know it is going to take at least two weeks to finish the work, but you aren't sure your boss realizes that. Or maybe he does and but he doesn't want to deal with the issue right now. In any case, it's a bad idea to say something like: "Are you out of your mind? There's no way we can deliver in a week." If it's not really a big deal, then don't say anything to your boss until after the meeting and let him handle it. If it is a big commitment and missing it will cause huge problems with the client, try saying (to your boss, not the client): "That seems like a very aggressive schedule. I think we can make it, but before we absolutely commit to it can we have some internal discussions?"
Also, never mention the names of other customers in a client meeting unless you are absolutely sure it is ok (like as a reference for proposed work). In one of my first jobs, I was at a company that sold financial planning tools to a bunch of firms on Wall Street who were huge competitors. They all needed to know that the work we were doing for them was strictly confidential. I was so nervous that in client meetings I would never even refer to the client by name, for fear that I would make a mistake and use the name of a competitor. Instead of saying "This software is just what Merrill Lynch needs", I'd say "This update is just what your company (or firm, etc.) needs." Call me paranoid but there are few things worse than mixing up client names.
You'll encounter all sorts of customers - some friendly and some mean and nasty. My advice is to think carefully and say little until you are comfortable in handling client situations.