Missed Supernova
I was checking through a list of recent supernova discoveries and realised that I had imaged one galaxy several times that contains a faint supernova that I did not spot when checking my images.
My image of 23rd February 2012 shows the supernova in the galaxy NGC 2755
The massive star (more than 10 times that of the Sun) that formed the Type II supernova had not exploded on 31st January 2012 as my image of that date shows.