Its not impossible to overcome but certainly its a problem when preconceptions enter into the mind of the experimentor. But then again, we could look at this from another angle. Any scientific inquiry that is made is first derived from some preconcieved notion that prompts an investigation of evidence. We call this a hypothesis. So, if you think about it, all hypothesis' begin with preconceptions.
You are using the word "preconception" equivocally. It is true that I must
think of an idea before I can test it; in that sense it is "preconceived". But there is no need for me to
believe that idea; which is the sort of "preconception" which might cloud the mind of an experimenter.