Labs! They have been the most overwhelming part of my career in chemistry. I felt the least prepared in this area when I began teaching and walked into my first lab as a teacher. Knowing all of the chemicals and equipment were under my care was a bit terrifying.
I am a safety conscience science teacher. I am embarrassed about some of the things that I did in my classes early in my career that I did not realize were unsafe. I saw the demonstrations and activities done at professional development venues and assumed that if my mentors were using the activity, it was safe.