Case in point: last year I was house managing for a youth symphony event. I was instructed to let in late arrivals at the end of each movement. I had at least 10 parents crammed into the alcove between the outer and inner doors so they could at least hear their little darling play. Every time, EVERY time there was a pause in the music I would reach for the door and one of the moms would murmur "Not yet."
This went on for at least 10 minutes, I shit you not. I still can't believe none of the parents took advantage of my ignorance to slip in early.