Expertise in the product you are selling is a key success factor to wining deals.  However, sales professionalism is still part of the overall process and simply having product knowledge does not guarantee a deal.  A common mistake made by the super knowledgeable sales person is to start rattling off all the virtues of their product or service without regard of customer need or requirement. This is one of primary sales crimes I teach new and experienced sales professionals.  Avoid the show up and throw up!

If you are selling a CRM system (happens to be what I do) and the customer calls in and asks for a demo.  You should always first spend some time with the prospect in discovery.  Go through the fundamentals and determine budget, authority, need and timeline (if your inside function has not already).  Then discuss their process and why they are looking at CRM.  Spend some time understanding their need and asking thoughtful questions about how they do business.  This process will give the sales person the proper framing for the upcoming presentation.  If the prospect is interested in customer service and you show them marketing automation you are not going to win the deal regardless of how well you know the marketing module.

Show up and throw up has many side effects.  The prospect will feel like you are not listening to them.  The customer will not respect the company or product or its representative and therefore the sales person will never achieve advisor status.

Product knowledge can get in the way of a sale.  But it is not the knowledge itself that is the problem. It is the inappropriate delivery of that knowledge that is the problem.