As a professional consultant or contractor your experience and skill in a particular set of technologies is one of the key reasons a company engages your services. However inevitably a question will occur that is outside of your knowledge. How you handle that question, in my opinion, defines a professional.
Depending on the individual, when a question arises that they don’t know the answer to, some people will resort to making something up. They may even become defensive and bluster when challenged or if additional questions occur.
I believe the correct response, and in fact the hallmark of a professional, is admitting that you don’t know and indicating you will find out. Importantly you need to then deliver on that promise.
But what about the alternative response?
Ignoring the obvious unprofessionalism of it, at some point the person will be found out. When that happens, it will call into question everything they have said, including the things they do know about.