About 15 years ago, when I was in school, it came time for finals. In one particular class (it was a PE class in Yoga, but that's not relevant to the story), the prof and the TA (a lovely girl) were proctering it, not unusual.
While I was taking the test, I heard the door to the room open, and I turned to see who was coming in. It was the TA.
I turned and looked towards the front of the room, and there was the teacher and the TA, confering. I look back, and there is the TA again, leaning against the wall, watching us.
It turns out that "the TA" was a pair of identical twins. They had been taking turns TAing the class. The prof knew about them, but nobody felt the need to tell the students.