One of the challenges of teaching is to explain a complex set of interrelated topics in a stepwise fashion. Often it is necessary to make reference to a concept that will be covered later, but without fully explaining it. Whilst this shorthand is useful, it is often not fulfilled in a rigorous fashion.
My suggestion is that teachers create list of all the things that they've said they'll cover off later. The list would be published to pupils and ticked off as covered. Each item on the list would be dated, and the context given. This would allow students to move backwards and forwards more easily in the syllabus.