What's the easiest type of question you have seen a colleague get wrong/not know how to do it?
This is not counting brief little mistakes, this is more of what's the easiest thing they got wrong but had declared that it was right, or were confident it was their answer (or option 3 admitted that they couldn't get an answer).
I have had one colleague get Pythagoras wrong when solving for a hypotenuse (he had been teaching for 40 years), and I had another coworker not know how to get find the height of an isosceles triangle if you only have the base and the length of the diagonals (she has been teaching maths for 12 years).