For me, it's my memory and lack of mindfulness. I will be training someone in the morning for a few hours and leave for lunch. Sometimes I run into them at lunch and I am panicked because I don't remember if they are the person I have been training today or someone I trained recently. (I work for a decent size company) I also had a couple of cases when I come back for lunch and was in a panic because I did not remember who I was training. This fortunately does not happen all the time. I try to be " mindful" but sometimes I just don't realize I haven't been until it's too late.