My students all knew they could drop in whenever they wanted - it was the standard within our department so it was nothing unusual for our students. Even given

My students all knew they could drop in whenever they wanted - it was the standard within our department so it was nothing unusual for our students. Even given this the vast majority never took advantage of this and I would usually only be visited by less than 5% of all students in a semester. Edit. In response to you edit this is very common. I had to teach all sorts of subjects at very short notice (the worst I was only given 3 days notice). I taught many subjects where the last contact I had had with the area was when I was an undergraduate. Makes for very stressful teaching.