Newton's Third Law
A syllabus I taught just a few years ago included the statement that 'candidates should show a good understanding of Newton's Third Law of Motion'. This made me smile, since I've known even quite experienced and well-qualified teachers get confused by it, so expecting pupils to get it right seemed optimistic at best.
I think the problem is partly that N(III) can seem, on the face of it, to be so straightforward as not to warrant any serious interest, yet it's full of potential pitfalls and is possibly one of the most misunderstood bits of school-level Physics. A while ago, I was asked to write a sheet explaining the problems that teachers encounter and how they can be avoided. Here it is: