I had it today when returning to St Nazaire after a pretty good patrol.
Time: Mid December '42
Location: about 400km out of St Nazaire
Sub: VIIC vs S-Class
I had only 2 TIV left in my aft tube, winds at 12 m/s so gun was out of question (at least for me...).
Here some pics:
First torpedo fired at 1600m:
1st torpedo homing in on the screws (although the sub was only making 8 knots...)
1st torpedo hits the screws but does not immobilize the sub...
So I set up for a second attack and see that the sub is listing already to the stern:
2nd and last torpedo fired and inbound...
Waiting for the impact..........AND
CLONK! 
A dud!
As I watched the sub with disgust through my periscope it opened fire with its deckgun and took 5% from my hull!
So I dove deep and crawled away...