Sometimes a common error, as people just have a rather ordinary interpretation on the meaning of the word "theory" and sometimes it's an intentional attempt of discrediting.

Words can mean different things in different contexts. A scientific theory is not the same as the general or ordinary every-day meaning of "theory".

Classic example and mistake by followers of creationist religions: "evolution is just a theory".

Well, what if I told you, that, for example, our modern electronic means of communication are part of the wide field of "information theory"?

Any argument that centers on the phrase "scientific theory" instantly goes in the trash and should tell you all you need to know about the person making it