This was one an entire class got away with.
Second year secondary school ( now year 8) 
End of year exams. We were sitting the maths paper first thing. Our form teacher, (a maths teacher) during registration, asked if there was anything we would like to quickly go over in the 20 minutes before the exam. We asked that she covered quadratic equations (which seemed to be the class bugbear) so she did.
The bell rang, teacher wished us luck and waltzed off to her next class leaving the solved equation on the board. We were taking the exam in our form room, which was unusual.
An English teacher (as in teacher of English) came in to invigilate and handed out the papers, warned us not to talk and then looked out of the window while we took the exam.
The very same quadratic equation was one of the questions. 😂
Exam over, we went to break and carried on our next exam.
After lunch, we went back to the form room for afternoon registration. Form teacher comes in, looks at the board with the solved equation clear to see, looks at the class, goes very red and quickly cleaned the board.
Needless to say that every one of us got the equation correct! 
Form teacher never mentioned it again.