A priori ideas and metaphors

One of the obstacles to understanding is the existence of incorrect a priori ideas and metaphors.
I offer a technical example:
  In TCP network programming there is an idea of a server and a client. People are often told that a good metaphor for a connection is a telephone call. They might then come to the conclusion that in a communication between two sockets neither is a server because it does not look like there is a server role when you ring someone: "both are in the same situation"

Another one, easier, concerns drag and drop operationson file icons in a Windowing interface.
In some cases it means copy and in other cases it means move. You can imagine that it is not a priori obvious that there should be this distinction.

Anybody teaching should try and identify potential a priori ideas so as to correct them.