Answers to this question will differ, of course, with the grade/age level addressed. Having worked at the secondary-level (i.e., grades 7-12), and now working with pre-service teachers in a teacher preparation program, I feel as though one of the biggest "mistakes" is a drift away from the systematic, rigorous lesson planning that is promoted in teacher preparation. We work tirelessly so that our pre-service teachers understand the value and importance of writing learning-centered objectives (Reeves, 2011), backward design (Wiggins & McTighe, 2005), alignment across all elements of lesson planning, etc. Unfortunately, the first few years of teaching are often overwhelming for many (if not all) new teachers, and the stress of their new positions frequently results in a search for the easiest way to plan lessons. I have seen many new teachers resort to simply photocopying worksheets since they are so overwhelmed with the profession of teaching in its entirety. I don't suggest by any means that this is ubiquitous, but it is a "mistake" that I have witnessed.
In any educational setting, students are our primary stakeholders. They are with us to learn. But, teachers tend to fall into some mistakes (knowingly or unknowingly);
- No clearly defined learning objectives
- Do not organize learning content to facilitate student learning
- Impose tasks/assignments that are not aligned to the learning outcomes
- Overlook the importance of formative assessments
A part of this can be attributed to the absence of formal teacher training in higher education settings.