In Richard Feynman's classic Cargo Cult Science speech. He talks about a type of scientific error made, where a person takes a previous experimental result for granted when designing an experiment.
"Other kinds of errors are more characteristic of poor science. When I was at Cornell, I often talked to the people in the psychology department. One of the students told me she wanted to do an experiment that went something like this--it had been found by others that under certain circumstances, X, rats did something, A. She was curious as to whether, if she changed the circumstances to Y, they would still do A. So her proposal was to do the experiment under circumstances Y and see if they still did A. I explained to her that it was necessary first to repeat in her laboratory the experiment of the other person--to do it under condition X to see if she could also get result A, and then change to Y and see if A changed. Then she would know the the real difference was the thing she thought she had under control."
I'm not sure I understand what sort of fallacy is being committed in this instance. What exactly is the problem?