After years of experience in some area of expertise, you can easily forget how difficult it can be for the uninitiated to grasp some fundamental concepts, and, indeed, people often edit out of their own personal history memories of their initial confusion and fears.
History can re-teach us about the inherent difficulties in assimilating new concepts, and, I think, help in tempering the novices' fears by assuring them that initial confusion and making mistakes are quite natural and in encouraging them to persevere.
Using the evolution of the concept of negative numbers as an example, I'd like to present a historical note from Mathematics: The Loss of Certainty by Morris Kline (Oxford, pg. 115):
"An interesting argument against negative numbers was given by Antoine Arnauld (1612-1694), theologian, mathematician, and close friend of Pascal. Arnaud questioned that -1 : 1 = 1 : -1 because, he said, -1 is less than +1; hence, how could a smaller be to a greater as a greater is to a smaller?"
The history of mathematics is replete with similar examples. What are some of your favorites?
Edit in response to comments:
I'm asking for historical examples that illustrate the natural confusion present in the evolution of fundamental concepts in math and that ideally could serve both to mitigate the remedial or novice learner's reaction to his own confusion and allied fear and as a stepping stone for exploring the concept.
(I know of other examples involving Leibniz and the product rule (cf. Humanizing Calculus by M. Cirillo), and Euler and log(-1), but I'll leave them for others to describe.)