Does anyone else have their favourite examples of things some teachers get wrong? Here are some that I have seen: 1. Getting the definitions of segment and sector the wrong way round (seen more than once) 2. Getting students to face the front, then turn 90 degree to the left, then another 90 degrees to the left, then another, then another, and saying that this demonstrates rotational symmentry of order 4. 3. Misapplication of BIDMAS/BODMAS to argue that addition takes precedence over subtraction, e.g. demonstrating to the class that 10 - 3 + 4 should be interpreted as 10 - (3 + 4) = 3, rather than working left to right as in 10 - 3 + 4 = 7 + 4 = 11. 4. Telling students that 0 is not a number because 'it is the absence of number' 5. Telling students that 1 is not a prime number because a prime number can be divided by itself and 1, but 1 can only be divided by itself. 6. Telling students that 1 is a prime number because it can be divided by itself and 1. Some are a bit more subtle: 7. Telling students triangle 'has' 180 degrees. 8. Talking about 'the function f(x)' rather than the function f. 9. Telling pupils that arcsin is the inverse of the sine function, rather than saying it is the inverse of the sine function with domain restricted to [-pi/2 , +pi/2]. If not convinced, try graphing y = arcsin(sin(x)) and comparing it with y = x. 10. Telling pupils that 'no-one knows what the biggest prime number is' rather than 'there is no biggest prime number' or that 1 - .9999...(recurring) is 'point 0 recurring with a one after it' 11. Telling students that an event is 'something that could happen' rather than a set of possible outcomes or a subset of the sample space, or that P(A|B) is the probability that A occurs given that B has already occurred 12. Having no answer to the question 'But what exactly is a random variable?' It strikes me that a collection of mistakes and confusions might be a useful resource for professional development.