This is a draft article for Hello World, the Computing pedagogy magazine from the Raspberry Pi Foundation. The full article will appear in a future edition, but you can also read much more about this topic in my book, available at Amazon here.
What is a Misconception?
A misconception is a view held by the learner at odds with true domain knowledge. They arise in everyday experiences, are imported from other subjects or are gained during prior learning episodes that were not fully understood.
In everyday conversation “the internet” is used to mean the world wide web, and that device provided by your ISP is not actually a “router”, it’s usually a combined router, switch, modem and wireless access point. Outside of networking, we find the misconception “secondary storage is used when primary storage is full” which seems plausible enough but wrong, and my students once tried to prank each other by suggesting slow performance could be solved by “downloading more RAM”.
Some misconceptions occur through misapplying learning in other subjects to Computing, using imperfect analogies to illustrate new concepts or by the deliberate teaching of a simplified truth lower down the school. The latter “necessary evil” misconceptions include “the computer understands binary” and “all programs are a series of steps”. Of course, the computer “understands” nothing, electrical circuits simply respond to high and low voltages. And students who study Computer Science to A-level will encounter object-oriented and functional programs which are not “a series of steps”.
Why are misconceptions important?
A 2016 study by Sadler et al showed that science teachers who were misconception-aware helped their students make better progress than those who were not. According to the researchers, “It is better if a teacher … has a model of how students tend to learn a particular concept, especially if a common belief may make acceptance of the scientific view or model difficult” 
Juha Sorva’s excellent 2018 paper “Misconceptions and the beginner programmer” can be found in “Computer Science Education” (Sentence et al, 2018).  As Sorva explains, “A student with [programming] misconceptions will produce programs that do not work. When a misconception persists, it can leave the student frustrated and unable to make progress.” (Sorva, 2018). 
Some key misconceptions.
Below is a selection of misconceptions from “How to Teach Computer Science”, from Sorva’s paper, crowdsourced my personal learning network or pulled from the “Project Quantum” set of diagnostic questions
|Data Representation||Binary can only represent numbers up to 255.||Binary is the base 2 number system and can represent any number.|
|Programming||While loops terminate the instant the condition turns false||A while loop condition is evaluated only on entry to the loop, and every time the code block inside the loop ends|
|Assignment statement a = b makes a and b always equal throughout the program execution||Assignment statements are made of two parts, the right-hand side of the assignment operator (=) is an expression which is evaluated, and the result stored in the variable on the left.|
|Architecture||Virtual Memory is Cloud Storage||Virtual memory is a space on the hard drive used to extend the available RAM|
|Logic||NOT is a binary operator, taking two inputs, for example in A NOT B||NOT is a unary operator, it takes only one argument, so|
A NOT B is invalid. NOT B is valid as is A AND NOT B.
|Networks||Peer-to-peer networks have a mesh topology||Network service model (P2P or client-server) is independent of the topology.|
|System Security||The “padlock” declares a website is trustworthy||The padlock next to the address bar in a browser indicates that that a secure connection has been established using HTTPS. This means only that traffic is secured against passive attack (eavesdropping) by third parties. It says little about the trustworthiness of the website.|
 Sadler et al 2016, “Understanding misconceptions”, https://files.eric.ed.gov/fulltext/EJ1094278.pdf
 Sentence, S et al (editors, 2018), “Computer Science Education”
 Sorva, J. 2018, “Misconceptions and the beginner programmer”
For much more on this subject, and a look at other pedagogical content knowledge useful to Computer Science teachers, see my book “How to Teach Computer Science” available now from publisher John Catt Educational, from Amazon and many online bookstores.