We, humans, have a tendency of following or believing those who are confident.  Sure, confidence is good, but it's also good to remember that not everything is always as it seems at first sight - Those overly confident peeps might also have something to hide. 

I’m talking about a cognitive bias amongst unskilled and unaware people. This bias, also known as the Dunning-Kruger effect, stops people from recognizing their own incompetence which leads to Inflated Self-Assessments. In other words, they think they're much more professional and capable than they ACTUALLY are… This incompetence becomes a problem if you’re THE PAYING CLIENT!

This works another way around as well… 

People with smarts and high education (I'm talking to you who are an expert in your field) often assume that tasks that are easy for them are also easy for other people.  

This little tidbit easy to forget when you have been doing your thing for years. But it is never okay to assume that all is clear because assuming often makes an ASS out of U & ME. Instead, ask and confirm. Asking and confirming is the most effective, as well as, the easiest way to make sure that your assignments are understood by your client. 

In conclusion

Next time, when a salesperson comes into your office guns blazing ready to lasso the moon, makes sure you don’t fall for the confidence act… At the same time, when you, salesperson or consultant, are helping your clients, ask if everything was understood and clear before you bolt to the next meeting.